Systems and methods for controlling and forming polymer gels

ABSTRACT

In preferred embodiments, the present invention provides methods of controllably making a vinyl polymer hydrogel having desired physical properties without chemical cross links or radiation. The gelation process is modulated by controlling, for example, the temperature of a resultant vinyl polymer mixture having a gellant or using active ingredients provided in an inactive gellant complex. In accordance with a preferred embodiment, the method of manufacturing a vinyl polymer hyrodgel includes the steps of providing a vinyl polymer solution comprising a vinyl polymer dissolved in a first solvent; heating the vinyl polymer solution to a temperature elevated above the melting point of the physical associations of the vinyl polymer, mixing the vinyl polymer solution with a gellant, wherein the resulting mixture has a higher Flory interaction parameter than the vinyl polymer solution; inducing gelation of the mixture of vinyl polymer solution and gellant; and controlling the gelation rate to form a viscoelastic solution, wherein workability is maintained for a predetermined period, thereby making a vinyl polymer hydrogel having the desired physical property. In further preferred embodiments, the present invention provides physically crosslinked hydrogels produced by controlled gelation of viscoelastic solution wherein workability is maintained for a predetermined period. In another aspect, the present invention provides kits for use in repairing intervertebral disks or articulated joints including components that form the vinyl polymer hydrogel and a dispenser.

CROSS REFERENCES TO RELATED APPLICATIONS

[0001] The present application is a continuation-in-part of U.S. patentapplication Ser. No. 10/631,491, filed Jul. 31, 2003 which claims thebenefit of U.S. Provisional Application No. 60/400,899, filed Aug. 2,2002, the entire contents of the applications being incorporated hereinby reference in their entirety.

BACKGROUND OF THE INVENTION

[0002] Lower back pain affects over 65 million people in the UnitedStates with an estimated 12 million of these cases arising fromdegenerative disk disease. The back is particularly susceptible todamage and disease due to its complex structure. The spine is a complexstructure of articulated bone and cartilage comprised of a column ofvertebrae separated by vertebral disks (FIG. 1). These vertebral disksact as an intervening cushion to mitigate and distribute loadstransferred along the spinal column.

[0003] The anisotropic structure of the intervertebral disk efficientlyachieves the appropriate mechanical properties required to cushioncomplex spinal loads. The inner viscoelastic material, termed thenucleus pulposus, occupies 20-40% of the total disk cross-sectionalarea. The nucleus usually contains between 70-90% water by weight. Thenucleus is composed of hydrophilic proteoglycans that attract water intothe nucleus and thus generate an osmotic swelling pressure rangingbetween 0.1-0.3 MPa, which supports the compressive load on the spine.The nucleus is constrained laterally by a highly structured outercollagen layer, termed the annulus fibrosus (FIG. 1). The nucleuspulposus is always in compression, while the annulus fibrosus is alwaysin tension. Although it comprises only one third of the total area ofthe disk cross-section, the nucleus supports 70% of the total loadexerted on the disk. The intervertebral disk becomes less elastic withage, reaching the elasticity of hard rubber in most middle-aged adultsas the nucleus loses water content. This water loss will also cause thedisk to shrink in size and will compromise its properties.

[0004] In degenerative disk disease, the nucleus pulposus can becomedistorted under stress, resulting in the extrusion of part of thepulposus out through the annulus fibrosus, causing pressure against thesurrounding nerves. This process is called herniation. The damage to thedisk can often be irreversible if part of the pulposus is lost. Themajority of disk injuries occur in the lumbar region, and the mostcommon area of disease occurs at L4/L5 and L5/S1.

[0005] A laminectomy (surgical removal of part of a herniateddisk—typically the nucleus pulposus) may be performed to relievepressure on local neural tissue. This approach is clearly a short-termsolution, given that the load bearing ability of the nucleus would bereduced with loss of material. Despite this, over 200,000 laminectomiesare performed each year, with a success rate of 70-80%.

[0006] Arthrodesis or fusion is a more permanent method for surgicallytreating degenerative disk disease. Fusion is accomplished with orwithout internal fixation. While internal fixation has becomeincreasingly popular, this technique has its share of complications.Fracture, neurological damage, and osteoporosis have been observed inpatients who have undergone internal fixation fusions. The ability ofthe bone to fuse will vary from patient to patient, with the averagelikelihood of success ranging from 75-80%. Spinal fusion will causestiffness and decreased motion of the spine. Additionally, fusion canalso put stress on adjacent vertebrae in the spine, which can acceleratedisease in adjacent disks and lead to additional back surgery.

[0007] A successfully designed artificial disk would replace a worn outdisk while protecting patients from incurring problems at an adjacentlevel of the spine. Several artificial disk prostheses have beenproposed in the prior art. Many of these prosthesis attempt completereplacement of the disk, including the nucleus and the annulus fibrosus.Given that the intervertebral disk is a complex joint withmulti-directional loading, the design of a prosthesis that mimics thearticulation and mechanical behavior of a natural disk isextraordinarily difficult. For example, when the body is supine,compressive loading on the third lumbar disk is 300 N, rising to 700 Nin an upright stance, then to 1200 N when bending forward by 20°.Additionally, moments of 6 N-m are often achieved during flexion andextension, with up to 5° of rotation. For adequate safety, a preferredcompressive strength of the entire disk is 4 MN/m².

[0008] The most extensive experience to date with a complete diskreplacement is that obtained with the SB Charité III prosthesis. Thisprosthesis has been used extensively in Europe since 1987, and has beenimplanted into over 3,000 patients. The SB III is designed with apolyethylene spacer placed between two cobalt chromium endplates. Twoyear follow-up studies have shown good clinical success in patients.Another study concerned a complete disk prosthesis consisting of apolyolefin core reinforced with carbon black, which is attached to twotitanium plates. Preliminary results are not promising, since the corefractured in 2 of the implants.

[0009] Both of the examples presented above serve to indicate that thereis considerable commercial effort being expended in the development ofartificial disks. However in both cases the mechanical equivalence ofthese components to the human intervertebral disks is somewhat doubtfuland the long-term clinical prognosis is still unclear.

[0010] As an alternative to the complete replacement of intervertebraldisks, the nucleus pulposus alone can be replaced, leaving the annulusfibrosus intact. This approach is advantageous if the fibrosis isintact, in that it is less invasive, and the annulus can be restored toits natural fiber length and fiber tension. In replacing the nucleus, itis desirable to find a material that is similar in properties to thenatural nucleus. The prior art describes bladders filled with air,saline, or a thixotropic gel. To prevent leakage, the membrane materialcomprising the bladder must be impermeable, which inhibits the naturaldiffusion of body fluid into the disk cavity, preventing access tonecessary nutrients.

[0011] To generate a more natural disk replacement material, severalresearch groups have investigated polymeric hydrogels as a possiblereplacement for the nucleus pulposus. Hydrogels are good analogs for thenucleus pulposus, in that they typically possess good viscoelasticproperties and can offer good mechanical behavior. Additionally, theycontain a large amount of free water, which permits a prosthesis madefrom a hydrogel to creep under compression and handle the cyclicalloading without loss of elasticity, similar to a natural nucleuspulposus. The water permeability of these materials also allowsdiffusion of body fluid and nutrients into the disk space. Control ofthis pore structure, and the consequent control of the nutrient accessto all parts of the implant, may be critical for future prostheticimplants.

[0012] Others have investigated the use ofpolyacrylonitrile-polyacrylamide multiblock copolymers encased in ajacket made from ultra high molecular weight polyethylene fibers. Thesesystems absorb up to 80% of their weight in water. Polyvinyl alcohol(PVA) and copolymers of PVA and poly vinyl pyrrolidone (PVP) haveproduced prostheses with mechanical properties similar to natural disks.These materials have the additional advantage of having clinical successin other medical devices. Gels formed from PVA are usually prepared viaa freeze-thaw process or via external crosslinking agents. In addition,hydrogel-based nuclei can contain therapeutic drugs which slowly diffuseout after implantation. Although no clinical data is currently availablefor these materials, biomechanical testing on cadaver joints has shownsimilar mechanical properties to natural disks.

SUMMARY OF THE INVENTION

[0013] In preferred embodiments, the present invention provides methodsof controlling the gelation kinetics of vinyl polymers. These methodsinclude, in preferred embodiments, controllably making a vinyl polymerhydrogel having desired physical properties without chemical cross linksor radiation. The gelation process is modulated by controlling, forexample, the temperature of a resultant vinyl polymer mixture having agellant (also spelled as gelant) or using active ingredients provided inan inactive gellant complex.

[0014] Preferred embodiments include an injectable hydrogel such as, forexample, for nucleus pulposus augmentation using minimally-invasivesurgical implantation of prosthetics fabricated in situ. The hydrogel inaccordance with preferred embodiments of the present invention conformto the region of interest, for example, vertebral surfaces in a jointspace. The load bearing hydrogels formed by in situ gelation methods ofthe present invention minimizes damage to, for example, the annulusfibrosus. In accordance with a preferred embodiment, the method ofmanufacturing a vinyl polymer hyrodgel includes the steps of providing avinyl polymer solution comprising a vinyl polymer dissolved in a firstsolvent; heating the vinyl polymer solution to a temperature elevatedabove the melting point of the physical associations of the vinylpolymer, mixing the vinyl polymer solution with a gellant, wherein theresulting mixture has a higher Flory interaction parameter than thevinyl polymer solution; inducing gelation of the mixture of vinylpolymer solution and gellant; and controlling the gelation rate to forma viscoelastic solution, wherein workability is maintained for apredetermined period, thereby making a vinyl polymer hydrogel having thedesired physical property. In further preferred embodiments, the presentinvention provides physically crosslinked hydrogels produced bycontrolled gelation of viscoelastic solution wherein workability ismaintained for a predetermined period. In another aspect, the presentinvention provides kits for use in repairing intervertebral disks orarticulated joints including components that form the vinyl polymerhydrogel and a dispenser.

[0015] In certain preferred embodiments, the step of providing a vinylpolymer solution typically includes the step of dissolving the vinylpolymer in the first solvent. The step of mixing the vinyl polymersolution with a gellant may precede or follow the step of heating thevinyl polymer solution to a temperature elevated above the melting pointof physical associations of the vinyl polymer.

[0016] The desired physical property typically includes at least one oflight transmission, gravimetric swell ratio, shear modulus, loadmodulus, loss modulus, storage modulus, dynamic modulus, compressivemodulus, cross-linking and pore size. In preferred embodiments, thedesired physical property is physical cross-linking.

[0017] In preferred embodiments, the vinyl polymer is selected from thegroup consisting of polyvinyl alcohol, polyvinyl acetate, polyvinylpyrrollidone and mixtures thereof. Preferably the vinyl polymer ishighly hydrolyzed polyvinyl alcohol of about 50 kg/mol to about 300kg/mol molecular weight. In preferred embodiments, the vinyl polymer ishighly hydrolyzed polyvinyl alcohol of about 100 kg/mol molecularweight. Typically the vinyl polymer solution is about 1 weight percentto about 50 weight percent solution of polyvinyl alcohol based on theweight of the solution. In preferred embodiments, the vinyl polymersolution is about 10 weight percent to about 20 weight percent solutionof polyvinyl alcohol based on the weight of the solution.

[0018] In certain preferred embodiments, the method includes the step offurther contacting the viscoelastic solution with a gellant, typicallyto modify the physical or chemical properties of the resulting gel. Thismethod is suitable for producing a local modification in the physicalproperties of the gel, such as to maintain the gel in place within abody cavity, such as a space within an intervertebral disk.

[0019] In preferred embodiments, the first solvent is selected from thegroup consisting of deionized water, dimethyl sulfoxide, an aqueoussolution of a C₁ to C₆ alcohol and mixtures thereof. Preferably thegellant is more soluble than the vinyl polymer. In certain preferredembodiments, the vinyl polymer is introduced into an aqueous solution ofa gellant.

[0020] Typically, the Flory interaction parameter of the mixture ofvinyl polymer solution and gellant ranges from 0.25 to 1.0. In preferredembodiments, the Flory interaction parameter of the mixture is at least0.5, more preferably about 0.25 to about 0.5.

[0021] Typically the gellant is selected from the group consisting ofsalts, alcohols, polyols, amino acids, sugars, proteins,polysaccharides, aqueous solutions thereof, and mixtures thereof. Inpreferred embodiments, the gellant is selected from the group consistingof chondroitin sulfate, dermatan sulfate, hyaluronic acid, heparinsulfate and mixtures thereof. In other preferred embodiments, thegellant is selected from the group consisting of biglycan, syndecan,keratocan, decorin, aggrecan and mixtures thereof. In further preferredembodiments, the gellant is an alkali metal salt, most preferably sodiumchloride.

[0022] The gellant may be added as a dry solid or in solution. Forexample, solid NaCl can be added to an aqueous solution of vinylpolymer, or added as an aqueous solution of sodium chloride from about1.5 molar to about 6.0 molar, more preferably about 2.0 molar to about6.0 molar. In further preferred embodiments, the gellant is an aqueoussolution of an alcohol chosen from the groups consisting of methanol,ethanol, i-propanol, t-propanol, t-butanol and mixtures thereof.

[0023] The gellant may be in an active form or an inactive form when itis mixed with the vinyl polymer solution. In preferred embodiments, thestep of inducing gelation of the viscoelastic solution includes the stepof activating the gellant. Preferably, the inactive gellant is activatedby a controllable trigger event.

[0024] In certain preferred embodiments, the inactive gellant is amacromolecule and the active gellant comprises fragments of amacromolecule that are released by cleavage of the macromolecule. Insome embodiments, the cleavage of the macromolecule is enzymaticcleavage; a preferred macromolecule is a physiological substrate of theselected enzyme. In preferred embodiments, the macromolecule is selectedfrom the group consisting of chondroitin sulfate, dermatan sulfate,keratan sulfate, hyaluronic acid, heparin, heparin sulfate and mixturesthereof and the enzyme is selected from the group consisting ofchondroitinase ABC, chondroitinase AC, chondroitinase B, testicularhyaluronidase, hyaluron lyase, heparinase I/III and mixtures thereof. Inother preferred embodiments, the macromolecule is selected from thegroup consisting of biglycan, syndecan, keratocan, decorin, aggrecan,perlecan, fibromodulin, versican, neurocan, brevican and mixturesthereof and the enzyme is selected from the group including, forexample, without limitation, aggrecanase and mixtures thereof.

[0025] In other embodiments, macromolecule can be thermally denaturable;in such embodiments, a preferred macromolecule is collagen.Alternatively, cleavage of the macromolecule is by irradiation withelectromagnetic radiation or particulate radiation.

[0026] In further preferred embodiments, the inactive gellant is a badsolvent sequestered in a vesicle, a liposome, a micelle or a gelparticle. In some preferred embodiments, the liposome is aphototriggerable diplasmalogen liposome. In alternate preferredembodiments, the liposome undergoes a phase transition at about the bodytemperature of a mammal. Preferably, the liposome includes, withoutlimitation, a mixture of dipalmitoylphosphatidylcholine anddimyristoylphosphatidylcholine.

[0027] In further preferred embodiments, the inactive gellant isassociated with a gel particle that is in an active form upon undergoinga phase transition at about the body temperature of a mammal. In suchembodiments, the gel particle suitably comprises a polymer selected fromthe group consisting of poly(N-isopropyl acrylamide-co-acrylic acid),N-isopropylacrylamide, hyaluronic acid, pluronic and mixtures thereof.In other preferred embodiments, the gel particle releases its contentsupon undergoing degradation.

[0028] Typically, the rate of gelation is controlled to provided anadequate period of workability needed for further processing of theviscoelastic solution, including injecting, molding or calendaring. Inpreferred embodiments, the viscoelastic solution is injected into anactual or potential space in the body of a mammal. In particularlypreferred embodiments, the viscoelastic solution is injected into anintervertebral disk or an articulated joint, such as a hip or knee. Thehydrogel can be retained within the body space by virtue of generalphysical properties or by its local physical properties at the injectionsite. In preferred embodiments, desired local physical properties can beadjusted by a further addition of gellant near the injection site. Inother preferred embodiments, the hydrogel can be retained within thebody space by the use of known medical devices to seal, reinforce orclose the injection site or other defect of the body cavity. Suitablesuch devices are disclosed in published International PatentApplications WO 01/12107 and WO 02/054978, which are hereby incorporatedby reference in their entirety.

[0029] In other preferred embodiments, the step of processing includescovering a burn or a wound.

[0030] The preferred embodiments of the present invention providemethods of making a gel and controlling a property of the gel. Inaccordance with a preferred embodiment of the present invention, amethod for making a gel includes comprising dissolving a vinyl polymerin a first solvent to form a vinyl polymer solution and introducing thevinyl polymer solution in a volume of a second solvent to causegelation, the second solvent having a higher Flory interaction parameterat a process temperature than the first solvent. The Flory interactionparameter (χ) is dimensionless and depends on, for example, temperature,concentration and pressure. Solvents can be characterized as having alow χ value or solvents having a higher χ value wherein χ=0 correspondsto a solvent which is similar to a monomer. A solvent having a higher χvalue is characterized as a solvent that causes a gelation process at atemperature. A thetagel, in accordance with the present invention, isformed by using a second solvent having a Flory interaction parameterthat is sufficient to cause gelation.

[0031] Preferably the second solvent used in the preferred embodimenthas a Flory interaction parameter in the range of 0.25 to 1.0.Typically, first and second solvent characteristics are chosen to allowuse of the method of the preferred embodiment at room temperature or atbody temperature of a mammal. The gel produced by the method of theinvention has physical cross-linking, and is substantially free ofchemical crosslinking agents. In a preferred embodiment, the vinylpolymer is polyvinyl alcohol.

[0032] In some embodiments, a plurality of cycles of contacting thevinyl in an immersion solvent (second solvent) and contacting with thefirst solvent are performed. Alternatively, the method may includesubjecting the gel to at least one freeze-thaw cycle. The polyvinylalcohol (PVA) hydrogels thus may be both a thetagel and a cryogel.Partial gelling can be accomplished with either method and thencompleted using the other, or even alternating between the two methods.

[0033] While the examples and discussion herein are directed towardsvinyl polymers and in particular PVA hydrogels, thetagels can be made ina similar manner using any polymer that possesses the appropriate kindof phase diagram as described hereinafter with respect to the Floryinteraction parameter. A mechanical force can be applied to the gelduring the gelling process, changing the manner in which it gels aridalternatively producing oriented gelation.

[0034] In several embodiments, the vinyl polymer is highly hydrolyzedpolyvinyl alcohol of about 50 kg/mol to about 300 kg/mol molecularweight. In other embodiments, the vinyl polymer is highly hydrolyzedpolyvinyl alcohol of about 100 kg/mol molecular weight. The vinylpolymer solution is about 1 weight percent to about 50 weight percentsolution of polyvinyl alcohol based on the weight of the solution.Preferably, the vinyl polymer solution is about 10 weight percent toabout 20 weight percent solution of polyvinyl alcohol based on theweight of the solution.

[0035] The first solvent is selected from the group of solvents having alow χ value that is not sufficient to enable gelation, i.e., solvents inwhich the energy of interaction between a polymer element and a solventmolecule adjacent to it exceeds the mean of the energies of interactionbetween the polymer-polymer and the solvent-solvent pairs, as discussedbelow. In several embodiments, the first solvent is selected, withoutlimitation, from the group consisting of deionized water, dimethylsulfoxide, an aqueous solution of a C₁ to C₆ alcohol and mixturesthereof.

[0036] In general, the immersion solution comprises a solvent having ahigh or sufficient χ value that enables gelation. In some preferredembodiments, the immersion solution is an aqueous solution of a salt ofan alkali metal, typically sodium chloride. In other embodiments, theimmersion solution is an aqueous solution of a C₁ to C₆ alcohol,typically an aqueous solution of an alcohol chosen from the groupsconsisting of methanol, ethanol, i-propanol, t-propanol, t-butanol andmixtures thereof. In certain embodiments, the immersion solution is anaqueous solution of methanol.

[0037] In general, the vinyl polymer gels of the present invention canbe made in-situ for applications such as filters, microfluidic devicesor drug release structures in situations in which freeze-thaw gelationmay be difficult or impossible to execute.

[0038] In one embodiment, the vinyl polymer solution is placed in achamber having at least two sides and a membrane. The membrane hasproperties that contain the vinyl polymer while providing access tosmall molecules and solvents.

[0039] In some embodiments, the vinyl polymer solution is separated bythe membrane from at least two different immersion solvents, typically afirst immersion solvent and a second immersion solvent. In someembodiments, the first immersion solvent is an aqueous solution ofsodium chloride from about 1.5 molar to about 6.0 molar. In someembodiments, the second immersion solvent is an aqueous solution ofsodium chloride from about 2.0 molar to about 6.0 molar. In otherembodiments, the first immersion solvent is a 1.5 molar aqueous solutionof sodium chloride and the second immersion solvent is an aqueoussolution of sodium chloride from about 2.0 molar to about 6.0 molar. Insuch embodiments, a gradient in chemical potential is formed across thevinyl polymer solution between at least two different immersionsolvents. In one embodiment, a gradient in chemical potential is formedacross the vinyl polymer solution of about 4 mol.cm⁻¹.

[0040] In general, a gradient of a property is formed across the vinylpolymer gel that corresponds to the gradient in chemical potentialformed across the vinyl polymer solution. Typically, the property is atleast one of light transmission, swell ratio, shear modulus, loadmodulus, loss modulus, storage modulus, dynamic modulus, compressivemodulus, cross-linking and pore size.

[0041] In some embodiments, one or both immersion solvents are changedin a temporal pattern to modulate the spatial gradient of a physicalproperty. Such temporal cycling is done on a time scale shorter than thediffusion time to make an inhomogeneous gel. In this way, gels can beproduced with a similar set of properties on the edges or peripheralregion and another set of properties in the central region, such asgreater cross-linking in the peripheral region as compared with thecentral region. Temporal cycling of immersion solvents can also be usedto modify the structure of the gel, for example, pore size, forproduction of filters. In such filters, small, locally varying pore sizemay be useful for some forms of chromatography (through size exclusion)or any other filtering application that requires pore size control.

[0042] Additional compounds can be combined in the physicallycross-linked gel, including but not limited to, ionic or non-ionicspecies such as hyaluronic acid, polyacrylic acid and therapeuticagents.

[0043] In one embodiment, the invention provides a physicallycross-linked hydrogel comprising at least about 10 weight percentpoly(vinyl alcohol) solution gelled by immersion in about 2 to about 3molar sodium chloride wherein the hydrogel is about 14 percent to about21 percent physically cross-linked. In such an embodiment the final gelcomprises about 12 to about 29 percent poly(vinyl alcohol).

[0044] The preferred embodiments of the present invention also providearticles of manufacture comprising a vinyl polymer gel having at leastone gradient of mechanical properties. PVA thetagels may be used as abiocompatible load bearing or non-load bearing material for replacement,repair or enhancement of tissue. In general, PVA thetagels can replacePVA cryogels in applications where PVA cryogels are used.

[0045] In one embodiment, a one-piece prosthetic intervertebral disk ismade comprising a polyvinyl polymer hydrogel wherein the distribution ofmechanical properties of the one-piece prosthetic intervertebral diskapproximates the spatial distribution of the mechanical properties ofthe combination of the nucleus pulposus and the annulus fibrosis of thenatural intervertebral disk.

[0046] High compression PVA thetagels can be made by placing PVA in areverse osmosis membrane with NaCl and then making the outsideconcentration of NaCl quite high to compress PVA/NaCl. The NaClconcentration will climb as water leaves the reverse osmosis membranegelling the PVA at high pressure. The concentration of PVA can bemodified by the ratio of NaCl to PVA inside the reverse osmosismembrane.

[0047] In a preferred embodiment, gel microparticles can be made throughgelation during agitation or by dropping blobs of fluid into acrosslinking solvent, such as the immersion solution.

[0048] In a preferred embodiment, gels can be embedded with particlesthat degrade (or do not adsorb) to “imprint” a pattern (“empty spaces”)on the gel or as the drug release centers. Embedding neutrally chargedpolymers of varying molecular weights can be used to “space fill” thegel. These polymers are removable after the process, leaving acontrolled pore structure. Materials that are sensitive to freeze-thawcycles can be encapsulated. The gels can be embedded with particles orpolymers that are electrostatically charged to provide extra repulsionat high compressions but are collapsed in high salt. Such embeddedparticles can be those that are active in some manner (e.g. forcatalyses). Hydroxyapatite particles or other osteoinductive particles,agents, and similar moieties can be embedded to encourage bony ingrowthfor possible cartilage replacement.

[0049] In a preferred embodiment, poly(vinyl alcohol) gels can be usedto contain and release bio-active compounds such as growth factors,fibronectin, collagen, vinculin, chemokines and cartilage includingtherapeutic agents. The teachings with respect to incorporation oftherapeutic agents of U.S. Pat. Nos. 5,260,066 and 5,288,503 are hereinincorporated in their entirety. Contained compounds such as therapeuticagents or drugs can be released over time to modulate the local growthof normal tissues such as bone, blood vessels and nerves or tumors.

[0050] Temporal modulation of immersion solvents can produce thetagelsin accordance with the preferred embodiment with appropriate structureand physical properties for containing and releasing drugs and otherbioactive molecules. In one embodiment, an outer skin is formed that ishighly crosslinked and an inner layer containing the drug/active agentis only weakly crosslinked. In such an embodiment, the outer skin is therate limiting component and has a constant release rate. Thus, drugrelease in accordance with a preferred embodiment includes the releaseprofile that is tunable by controlling the spatial gradient in PVAcrosslinking.

[0051] In one embodiment, the present invention provides a method ofcontrollably modulating the mechanical properties and structure ofhydrogels. In a preferred embodiment, the present invention providesarticles of manufacture with one or more gradients of mechanicalproperties that more closely match the existing gradients of suchproperties in natural structures. In one embodiment the inventionprovides prosthetic hydrogel articles of manufacture that mimic themechanical behavior of natural structures. In a preferred embodiment,the invention provides polyvinyl alcohol prosthetic intervertebral disksthat mimic gradients of mechanical properties found in the naturalintervertebral disks. In another preferred embodiment, the inventionprovides a one-piece prosthetic intervertebral disk that mimics thespatial distribution of the mechanical properties of the nucleuspulposus plus annulus fibrosis of the natural intervertebral disk.

[0052] In a preferred embodiment, particulates may also be added to thegel. As described hereinbefore, particulates can be added to create acontrolled pore structure. Further, in accordance with another preferredembodiment, particulates can be added to provide a particularnanostructured gel. The particles can be either charged or uncharged andallow PVA crystals to nucleate at the surface of the particles.Particles that can be added include, but are not limited to, inorganicor organic colloidal species such as, for example, silica, clay,hydroxyapatite, titanium dioxide or polyhedral oligomeric silsesquioxane(POSS).

[0053] In accordance with another preferred embodiment, particles areadded to provide a charge effect to change the compressive modulus ofthe gel, and preferably increase the compressive modulus. Thisembodiment can use a thetagel having added particles. Upon compressingthe gel in a salt solution, a structure having particles with closepacking while shielding their charges results. Upon rehydrating with,for example, deionized (DI) water, the charge fields expand and resultsin a gel in tension. This allows the gel to approximate physicalproperties of cartilage, for example, at high charged particulate loads.

[0054] In accordance with another preferred embodiment, particulates areadded to the gel structure to provide mechanical properties such as, forexample, wear resistance. The addition of hardened glass (silica) ordifferent clays can provide wear resistance to the gels.

[0055] In accordance with another preferred embodiment of the presentinvention, a method for making a gel and controlling a property of thegel includes forming a thetagel as described hereinbefore by using afirst solvent to form a vinyl polymer solution and subsequentlyintroducing a volume of a second solvent to cause gelation, followed bypromoting dehydration to controllably structure the gel. This methodresults in uniformly structuring the gel and homogenizing the physicalcrosslinking of the PVA thetagel. This structure can be achieved byimmersing the contained PVA solution into a solvent which has a Floryinteraction parameter that is higher than the theta point for the PVAsolvent pair, and subsequently immersing the contained PVA in anothersolvent having a Flory interaction parameter lower than the theta pointfor the PVA solvent pair. The process can continue with successivedecreases in the Flory interaction parameter until the desiredinteraction parameter value for the gel is reached.

[0056] In another method in accordance with a preferred embodiment ofthe present invention, the PVA solution can be subjected to a graduallychanging solvent quality through a similar range of electrolyteconcentrations by the gradual addition of a concentrated NaCl solutionto deionized water such that the change of the salt concentration isslower, or equal, to the diffusion process of the gel.

[0057] In accordance with another preferred embodiment of the presentinvention, the PVA solution may be subjected to at least one freeze-thawcycle to fix the gel into a particular shape and then be immersed in aseries of solutions with successively higher Flory interactionparameters until the final desired Flory parameter is reached.Alternatively, the PVA solution is subjected to the one or morefreeze-thaw cycles after being immersed in a solution of 2 M NaCl.

[0058] In one preferred embodiment, nanoparticles are dispersed intosolutions of PVA. The solvent may be water, dimethyl sulfoxide (DMSO),methanol or any other solution that exhibits a Flory interactionparameter that is lower than the theta point for the PVA solvent pairduring solution preparation. The PVA/nanoparticle mixture is thensubjected to at least one freeze-thaw cycle. Subsequent to thefreeze-thaw cycling, the gelled PVA is immersed in a solvent that has aFlory interaction parameter near or higher than the theta point for thePVA/solvent pair to induce further physical crosslinking of thePVA/nanoparticle mixture.

[0059] Another aspect of the embodiments of the present inventionfurther provide methods of controlling the rate of gelation of polymergels by changing in the manner in which the polymer molecules interact.By controlling the rate of gelation, a “window” of time that allows therelatively slowly gelling PVA solution to be manipulated or worked, forexample, by injection, molding or any other processing step that may bedependent on the flow of the gelling PVA solution. In preferredembodiments, the gelling PVA solution is injected into a region ofinterest such as a body cavity and gels in situ to form a PVA product.Preferred body cavities include the nucleus pulposus and a normal orpathological void within a joint.

[0060] In preferred embodiments, the rate of gelation can be controlledby holding the polymer, preferably PVA, above its crystallizationtemperature, thus preventing gelation even if the solvent quality ispoor. In other preferred embodiments, the rate of gelation can becontrolled by using a second solvent that can be triggered to changefrom good to bad. In some preferred embodiments, the quality of thesolvent can be changed by disruption of micelles. In other preferredembodiments, the quality of the solvent can be changed scission of highmolecular weight molecules. In further preferred embodiments, a poorsolvent is used in combination with process temperatures that acceleratethe gelation process.

[0061] The foregoing and other features and advantages of the systemsand methods for controlling and forming polymer gels will be apparentfrom the following more particular description of preferred embodimentsof the system and method as illustrated in the accompanying drawings inwhich like reference characters refer to the same parts throughout thedifferent views. The drawings are not necessarily to scale, emphasisinstead being placed upon illustrating the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0062]FIGS. 1A and 1B are a diagrammatic representation of spinalanatomy showing transverse process, spinous process and vertebral bodyof the vertebral bones, the spinal cord and spinal nerves, and thenucleus pulposus of the intervertebral disks. The annulus fibrosis ofthe intervertebral disk surrounds the nucleus pulposus on lateral,anterior and posterior sides.

[0063]FIG. 2 is a graphical representation of the relationship of theFlory interaction parameter, χ, to the concentration (Φ) of a polymer ata given temperature in accordance with preferred embodiments of thepresent invention. The abscissa, at χ=0.25, separates polymer solutionsin first solvents (below) and polymer solutions in second solvents(above). The arrows and diamonds indicate the effect on χ produced byreplacing solvent 1 with solvent 2.

[0064]FIG. 3 shows 10% PVA solution in dialyzer cassettes after 1 day(top) and 3 days (bottom) of immersion in curing solution in accordancewith a preferred embodiment of the present invention. From left toright: 1.5 M NaCl, 2.0 M NaCl and 3.0 M NaCl. The 1.5 M solution doesnot gel the PVA, the 2.0 M solution and 3.0 M solution do gel the PVA.Note the progressive opacification of the 2.0 M gel and the shrinkage ofthe 3 M gel from the edges of the cassette as the sample compacts withtime (indicated with arrow).

[0065]FIG. 4 shows a uniform thetagel in accordance with a preferredembodiment of the present invention. PVA gels generated by immersion in3.0 M (left image of each pair) and 2.0 M (right image) NaCl curingsolution. Note that the gels are uniform and opaque. The gel exposed to3.0 M NaCl swells less and is more compact following equilibration indeionized water.

[0066]FIG. 5 shows an example of a gradient gel in accordance with apreferred embodiment of the present invention using a 10% PVA solutionexposed to spatially varying NaCl concentration. Note the variation inboth the translucency of the gel and in the swelling ratio.

[0067]FIG. 6 shows a differential scanning calorimetry (DSC) thermogramcomparing the results obtained with a thermally cycled PVA cryogel andthe “thetagel” in accordance with a preferred embodiment of the presentinvention. The solid line is indicative of 10% PVA immersed in 3.0 MNaCl for 3 days; the dashed line is indicative of 10% PVA thermallycycled 4 times from 10 degrees Celsius to −20 degrees Celsius with awarming rate of 0.02 degrees Celsius/min.

[0068]FIG. 7 graphically illustrates the relationship between thepercentage of PVA in PVA hydrogels that were fully equilibrated indeionized water after being gelled in immersion solutions of differentmolarities in accordance with a preferred embodiment of the presentinvention. The connected points represent measurements of 10% PVAimmersed for 3 days, the single point represents an initial solution of20% PVA solution immersed in 3 M NaCl for 12 days. For the 20% PVAsolution, the 3 day value of swelling ratio and percentage of PVAmatched that of the 10% PVA solution (not shown). After 12 days ofimmersion in 3 M NaCl (and 5 days of equilibration in deionized water),the 20% PVA solution formed a gel that was 29% PVA.

[0069]FIG. 8 graphically illustrates the gravimetric swelling ratio forPVA hydrogels that were fully equilibrated in deionized water afterbeing gelled in immersion solutions of different molarities inaccordance with a preferred embodiment of the present invention. Theconnected points represent measurements of 10% PVA immersed for 3 days,the single point represents an initial solution of 20% PVA solutionimmersed in 3 M NaCl for 12 days. For the 20% PVA solution, the 3 dayvalue of swelling ratio and percentage of PVA matched that of the 10%PVA solution (not shown).

[0070]FIG. 9 shows the dynamic modulus of PVA thetagel at 3 M NaCl, 20%initial PVA concentration and 1 N static load versus aging time in daysin accordance with a preferred embodiment of the present invention.

[0071]FIG. 10 shows the complex modulus (Storage (G′) and Loss (G″)Modulus) of PVA thetagel at 20% initial PVA concentration and 0.25 Nstatic load against solution molarity (2M and 3M NaCl) in accordancewith a preferred embodiment of the present invention.

[0072]FIG. 11 is a schematic diagram of an “Ussing” type chamber used tocreate a gradient gel 160 in accordance with a preferred embodiment ofthe present invention.

[0073]FIG. 12 illustrates a quick freeze deep etch (QFDE) image of PVAgel structure in accordance with a preferred embodiment of the presentinvention wherein the PVA gel is formed by immersion in 5 M NaCl for 3days. The bar represents 100 nm.

[0074]FIGS. 13A and 13B are a cross-sectional and a close-up view of thecross-section of a PVA gradient hydrogel, respectively, prepared byfilling Plexiglass tubing with 10% PVA solution, performing one freezethaw cycle (8 hours at −21° C.; 4 hours at room temperature) thenimmersing in 3 M NaCl bath for at least 3 days, then dehydrating in airfor 60 hours and returning to deionized (DI) water in accordance with apreferred embodiment of the present invention.

[0075]FIG. 14 illustrates a cross-sectional view of a PVA gradienthydrogel prepared by filling dialysis cartridge with 10% PVA solution,then immersing in a chamber having 3 M NaCl on one side and 6 M NaCl onthe other side for 3 days in accordance with a preferred embodiment ofthe present invention.

[0076]FIG. 15 illustrates a close-up view of the cross-section of thePVA gradient hydrogel of FIG. 14 on the 6 M NaCl side prepared byfilling the dialysis cartridge with 10% PVA solution, and then immersingin a chamber with 3 M NaCl on one side and 6 M NaCl on the other for 3days.

[0077]FIG. 16 illustrates a nanostructured PVA hydrogel prepared bymixing a solution of 10% PVA and 2 weight percent Laponite clay,subjecting to a 1 freeze-thaw cycle, then exposing to a 4 M NaClsolution for at least 3 days in accordance with a preferred embodimentof the present invention.

[0078]FIG. 17 illustrates a nanostructured PVA hydrogel prepared bymixing a solution of 10% PVA and 4 weight percent of silica, titratingto pH=3, subjecting to a 1 freeze-thaw cycle, then exposing to a 4 MNaCl solution for at least 3 days in accordance with a preferredembodiment of the present invention.

[0079]FIG. 18 illustrates a nanostructured PVA hydrogel prepared bymixing a solution of 10% PVA and 4 weight percent silica, titrating topH=10, subjecting to 1 freeze-thaw cycle, then exposing to 4 M NaClsolution for at least 3 days in accordance with a preferred embodimentof the present invention.

[0080]FIG. 19 illustrates a nanostructured PVA hydrogel prepared bymixing a solution of 10% PVA and an octatetramethylammonium polyhedraloligomeric silsesquioxane (OctaTMA POSS) in water, then subjecting to 1freeze-thaw cycle in accordance with a preferred embodiment of thepresent invention.

[0081]FIG. 20 graphically illustrates the storage modulus for hybrid andcontrol PVA gels in accordance with a preferred embodiment of thepresent invention.

[0082]FIG. 21A illustrates a flow chart of a method of forming a PVAhydrogel in accordance with a preferred embodiment of the presentinvention.

[0083]FIG. 21B illustrates a flow chart of methods of forming andproviding a vinyl polymer hydrogel in accordance with preferredembodiments of the present invention.

[0084]FIGS. 22A-22D illustrate a PVA hydrogel prepared by adding 1.4 gof 400 molecular weight poly(ethylene glycol) (PEG 400, Sigma Aldrich)to 6 g of an aqueous 10 wt % PVA solution while mixing, in accordancewith a preferred embodiment of the present invention, showing theproduct at four time durations after the end of mixing: FIG. 22A, zerominutes; FIG. 22B, 15 minutes; FIG. 22C, 2 hours, under a mineral oilprotective layer; and FIG. 22D, one day, out of a jar.

[0085]FIGS. 23A-23E illustrate a PVA hydrogel prepared by adding 35.6 gof aqueous 10 wt % PVA solution to 18.7 g of aqueous 5.1 M NaCl whilemixing, and the resulting mixture aggressively shaken, in accordancewith a preferred embodiment of the present invention, showing theproduct at five time durations after pouring into a covered dish and aflexible bag: FIG. 23A, zero minutes; FIG. 23B, 20 minutes; FIG. 23C, 1hour; FIG. 23D, 2 hours; and FIG. 23E, 17 hours.

[0086]FIGS. 24A-24F illustrate a PVA hydrogel prepared by adding NaCl toaqueous 10 wt % PVA solution at about 95 degrees Celsius (FIG. 24A)while mixing to make a final concentration of 2M NaCl (FIG. 24B), inaccordance with a preferred embodiment of the present invention. Aftermixing for 15 minutes, aliquots of the resulting mixture were pouredinto two containers that were cooled at room temperature (FIG. 24C, 15minutes; FIG. 24E, one hour) or on shaved ice (FIG. 24D, 15 minutes;FIG. 24F, one hour).

[0087]FIGS. 25A-25F illustrate the PVA hydrogels of FIGS. 24A-24F afterstorage. FIG. 25A, cooled one hour at room temperature and stored 12hours at room temperature; FIG. 25B, cooled one hour on ice and stored12 hours at room temperature; FIG. 25C, cooled one hour at roomtemperature and stored one month at room temperature; FIG. 25D, cooledone hour on ice and stored one month at room temperature; FIG. 25E, thePVA gel of FIG. 25C, oriented to show water released due to syneresis;and FIG. 25F, the PVA gel of FIG. 25D, oriented to show water releaseddue to syneresis.

[0088]FIGS. 26A-26D illustrate a PVA hydrogel prepared by adding NaCl toaqueous 10 wt % PVA solution at 95 degrees Celsius while mixing to makea final concentration of 2.1M NaCl, in accordance with a preferredembodiment of the present invention, wherein FIG. 26A shows a moldformed by a chilled polyethylene liner and the matching ball from atotal hip replacement joint, FIG. 26B shows the mold after filling thechilled polyethylene liner with the PVA solution and putting thematching ball in place, FIG. 26C shows the molded PVA in thepolyethylene liner after one hour in air at room temperature followed byone hour in deionized water at room temperature; and FIG. 26D shows themolded PVA product removed from the polyethylene liner.

[0089]FIGS. 27A-27B illustrate PVA hydrogels incorporating chondroitinsulfate (CS) prepared in accordance with preferred embodiments of thepresent invention, where FIG. 27A shows a PVA hydrogel formed by addingwarm CS solution at about 80 degrees Celsius to a aqueous 10 wt % PVAsolution at about 60 degrees Celsius to produce a 5 wt % PVA, 7 wt % CSmixture and FIG. 27B shows a PVA hydrogel formed by adding 600 mg CSdirectly to 10 ml aqueous 10 wt % PVA solution.

[0090]FIGS. 28A-28C illustrate flow charts of methods for forming a PVAhydrogel in accordance with preferred embodiments of the presentinvention.

[0091]FIGS. 29A-29F schematically illustrate a method for forming anddispensing a vinyl polymer hydrogel in accordance with a preferredembodiment of the present invention.

[0092]FIGS. 30A-30F schematically illustrate a method for forming anddispensing a vinyl polymer hydrogel in accordance with an alternatepreferred embodiment of the present invention.

[0093]FIGS. 31A-31E schematically illustrate a method for forming anddispensing a vinyl polymer hydrogel in accordance with an alternatepreferred embodiment of the present invention.

[0094]FIG. 32 schematically illustrates a dispenser for providing avinyl polymer hydrogel in accordance with a preferred embodiment of thepresent invention.

[0095]FIG. 33 illustrates a midsagittal cross-section of a portion of afunctional spine unit in which two vertebrae and the intervertebral diskare visible.

[0096]FIG. 34 illustrates a transverse section through a damagedintervertebral disk, showing a spinous process and transverse process ofan adjacent vertebra, annulus fibrosis and, nucleus pulposus.

[0097]FIG. 35 illustrates a step of dispensing a mixture of gellant andvinyl polymer in a method for repairing a damaged intervertebral disk inaccordance with a preferred embodiment of the present invention.

[0098]FIG. 36 illustrates a step in a method for repairing a damagedintervertebral disk in accordance with a preferred embodiment of thepresent invention showing, in particular, a mixture of gellant and vinylpolymer, a sealant, a fixative, and the fixative delivery instrument.

[0099]FIG. 37 illustrates a step in a method for repairing a damagedintervertebral disk in accordance with a preferred embodiment of thepresent invention showing, in particular, a mixture of gellant and vinylpolymer, barrier, a fixative, and a fixative delivery instrument.

[0100]FIG. 38 illustrates a step in a method for repairing a damagedintervertebral disk in accordance with a preferred embodiment of thepresent invention showing, in particular, the mixture of gellant andvinyl polymer substantially filling the space previously occupied by thenucleus pulposus, a barrier, and a dispensing tube.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0101] The preferred embodiments of the present invention are directedat the generation of uniform PVA hydrogels without chemical crosslinksor irradiation to produce a biocompatible material suitable for use as,for example, an intervertebral disk prosthesis. Further, a preferredembodiment of the present invention includes a method used to create thePVA gels that result in a new class of PVA hydrogels which can bedesigned for specific applications to have a potentially large range ofmechanical properties, being controllable, and which can be engineeredwith gradients in structure and physical properties.

[0102] As used herein, “theta solvent” refers to a solvent that yields,at the theta temperature, solutions of a polymer in the theta state.Theta solvents may be composed of a single solvent or mixture of twosolvents, a mixture of a solvent and a nonsolvent, or even a mixture oftwo nonsolvents in the case of co-solvency as described by Elias, H. G.,“Theta Solvents,” in Brandrup, J. and E. H. Immergut, Polymer Handbook₃rd Ed., John Wiley and Sons, New York, 1989, the teachings of which areherein incorporated by reference in their entirety.

[0103] As used herein, “gelation” refers to the formation of permanentphysical cross-links due to the crystallization of the PVA. This is incontrast to some of the literature in which gelation refers to the pointat which the polymer associates, and need not be followed by thecrystallization that forms a permanent link.

[0104] As used herein, “associate” refers to a thermodynamically, orsolvent, driven process whereby the polymer (or other species) “prefers”to be in an environment similar to itself, rather than in closeproximity to another species. This association of the polymer locallythen necessarily leads to a “phase separation” that may or may not leadto an inhomogeneous solution.

[0105] While not being held to a particular theory, it is thought thatforcing poly(vinyl alcohol) polymer chains in solution into closeproximity using a theta solvent through a spinodal decompositionmechanism results in the formation of a physical association that isresistant to dissolution. As used herein, spinodal decomposition refersto a clustering reaction in a homogeneous, supersaturated solution(solid or liquid) that is unstable against infinitesimal fluctuations indensity or composition. The solution therefore separates spontaneouslyinto two phases, starting with small fluctuations and proceeding with adecrease in the Gibbs energy without a nucleation barrier.

[0106] The methodology used in the present invention generates a PVAhydrogel employing the controlled use of solvents having a χ valuesufficient to cause gelation to force the PVA chains to physicallyassociate. To prevent random “crashing out” of the PVA, it is criticalthat the solvent quality is controlled carefully, and, in particular forlarger components, that the solvent “front” enters the PVA solution in acontrolled manner. For example, NaCl/deionized (DI) water andmethanol/deionized water solutions at temperatures and concentrations inthe neighborhood of their “theta” value for PVA were used to force thephysical association and subsequent gelling of the PVA. Gels formed inthis way are called “thetagels” herein.

[0107] In general, by controlling solvent quality one can create a“window” of time that allows the relatively slowly gelling PVA solutionto be manipulated or worked. This manipulation includes, withoutlimitation, injection or molding or any other conceivable processingstep.

[0108] The methods are applicable to the creation of materials for usein medical, biological and industrial areas including the controlleddelivery of agents (which may include proteins, peptides,polysaccharides, genes, DNA, antisense to DNA, ribozymes, hormones,growth factors, a wide range of drugs, imaging agents for CAT, SPECT,x-ray, fluoroscopy, PET, MRI and ultrasound), generation of load bearingimplants for hip, spine, knee, elbow, shoulder, wrist, hand, ankle, footand jaw, generation of a variety of other medical implants and devices(which may include active bandages, trans-epithelial drug deliverydevices, sponges, anti-adhesion materials, artificial vitreous humor,contact lens, breast implants, stents and artificial cartilage that isnot load bearing (i.e., ear and nose)), any application where gradients(single or multiple) in mechanical properties or structure are required.

[0109] The mechanism of gel formation includes a phase separationusually considered to be a spinodal decomposition process followed by acrystallization mediated by hydrogen bonding in the PVA rich regions ofthe solution. The choice of the solvent is known to influence thefreeze-thaw process. It is also known that some solvents can be used togel PVA without dropping the temperature of the solution below thefreezing point of the solvent. Solvents such as DMSO, ethylene glycol(EG), N-methyl-2-pyrrolidone (NMP), and gelatin have been used to gelPVA in ambient temperatures. The preferred embodiments of the presentinvention include the use of salt as a means of gelling a PVA solution,and the utility of manipulating and controlling the solvent qualityduring the gelation process.

[0110] An ideal polymer chain in a good solvent has no associationbehavior that leads to precipitation. As the solvent quality drops belowthe theta condition, the chains begin to prefer to associate untileventually the polymer phase separates. In an ideal polymer, there areno further interaction forces in the polymer chains, other than thatprovided through thermodynamic (solvent) forces, and the normal,reversible phase diagram thus applies. However, in PVA, there is astrong binding force driven by hydrogen bonding that results in a localcrystallization of the chain. This crystallization can occur even whenthe solvent is “good.” Although the chemical, potential in a goodsolvent does not encourage association, random thermal motions do allowthe chains to briefly contact and occasionally overcome this “solvationforce”. At this point, if the conditions are correct, as is the case forPVA, the chains will start to crystallize. This rate of initial contactto initiate crystallization depends on the solvent quality. Therefore,as the solvent quality is decreased, the probability of the PVA chainassociating increases since this is driven by the affinity of the chainfor itself, which is described by the solvent quality. As a result, rateof change of solvent quality is important, as has been observed bymanipulation of temperature. Since this process is related to thesolvent quality, it depends on temperature, concentration and pressure.Therefore, the gelling of the PVA must depend on time, and moreimportantly, a competition between the association “time” and the phaseseparation “time”. This competition has been observed experimentally,resulting in different initial gel structures depending on cooling ratesand solvents, and in the aging of PVA gels.

[0111] Therefore controlling solvent quality by controlled solventaddition, or by temperature, pressure or any other relevant parameterallows one to control the rate of this association. Consequently, onedesires to control the solvent quality of the system such that it ispoor enough to accelerate the association rate by promoting theproximity of the chains while ensuring that the solvent is not so badthat the polymer falls out of solution before crystallization can occur.The optimal conditions for gel formation are likely to be in the rangeof χ=0.25 to 0.5, but even outside these values it is possible toproduce gels. By balancing these competing effects, a polymer/solventsolution can be obtained that is still fluid but gels rapidly.

[0112] For a single solvent with its solvent quality varied bytemperature, or pressure, control, the spinodal decomposition phasetransition is accelerated by the decrease in solvent quality. For aternary blend, the phase separation can be understood as either the PVAis poorly solvated by the (continuous) water/solvent B blend and thusphase separates under the identical mechanism as the single solventsystem; or both the PVA and solvent B are solvated by water, but havevarying miscibility with each other. These materials therefore behavelike a binary blend and, therefore, phase separate. Althoughconceptually slightly different, the results are almost identical. Thebehavior of two good solvents that result in a poor solvent is known asco-nonsolvency. This approach provides the ability to mix the componentsexternally and then inject the PVA solution into a region of interestsuch as a cavity wherein it subsequently gels. This ability to injectPVA and induce gelation in situ is especially suitable for nucleusreplacement or augmentation of intervertebral disks (IVD), since thesecond solvent can be anything that has the required thermodynamicproperties. This second solvent can be a long chain polymer, protein orother complex molecule, or an aqueous solution thereof, such aspolyethylene oxide (PEO), a glycosaminoglycan (GAG) or gelatin, orsimpler molecules such as simply ionic species like salts, for example,NaCl or short chain molecules like poly(ethylene glycol), for example,PEG 400, glycerin or amino acids like serine or glycine. Although highermolecular weight species are not commonly characterized as solvents, theunderlying theory, as described by Flory, makes no distinction as to thelength of the solvent molecule, although the final interaction parameterfor the solvent/polymer pair is affected by the solvent size.

[0113] PVA is known to associate at room temperature in deionized (DI)water over long times. This is well below the theta transition of PVA inpure water of 97° C. The crystallites generated in both the freeze-thawprocess, and the thetagel process as used herein have a melt peak near60° C. In addition, most studies involving PVA require elevatedtemperatures of at least 80° C. to ensure that the PVA is fully“dissolved”. These observations suggest that above this temperature thePVA no longer possesses the ability to crystallize, and hence behaveslike an ideal polymer with respect to solvent quality. Consequently, itis recognized herein that if one can dissolve the PVA in a solutionabove this critical melt temperature there can be no crystallization,and hence no gelation, although there may be precipitation, or phaseseparation, due to simple association. If the solvent used is poor thenthere is some association through thermodynamic effects (and thusinhomogeneous precipitation may occur), but there can be no irreversiblegelation that characterizes the PVA hydrogels. However, as soon as thetemperature is lowered, crystallization can occur and the system canbegin to gel. This gelation crystallization is still however ratelimited, thus allowing the solution to be workable, even whilst belowthis temperature, and in a poor solvent. Mixing may influence thisgelation rate by encouraging collision of the polymer chains.

[0114] In general, a physically cross-linked poly(vinyl alcohol) gel isprepared from an aqueous poly(vinyl alcohol) solution (from 1% to 50%PVA by weight of the solution) that is gelled by contacting with asolvent having a χ value sufficient for gelation, hereinafter called thesecond solvent at a concentration approximating the “theta”concentration for the poly(vinyl alcohol) solution.

[0115] The present invention provides methods of producing poly(vinylalcohol) hydrogels that do not use chemical cross-linkers, irradiationor thermal cycling. In preferred embodiments, the solvent quality iscontrolled, preferably by controlling the diffusion of the secondsolvent (NaCl or methanol) into a PVA solution to produce a homogenous,physically crosslinked structure.

[0116] The present method uses a controlled change in solvents differingin solvent quality, conveniently expressed by the Flory interactionparameter to force the PVA to associate. Because no chemicalcross-linkers are used, the gel is substantially free of chemicalcrosslinkers and thus likely to be as biocompatible as thermally-cycledPVA cryogels. Any residue of NaCl in the gel following equilibration indeionized water is likely to be benign, as its concentration willcertainly be below physiologically relevant values.

[0117]FIG. 2 illustrates the relationship between the first solvent andthe second solvent in terms of the Flory interaction parameter, χ. FIG.2 is a graphical representation of the relationship of the Floryinteraction parameter, χ, to the concentration (Φ) of a polymer at agiven temperature. The abscissa, at χ=0.25, separates polymer solutionsin first solvents (below) and polymer solutions in second solvents(above) sufficient to cause gelation. The arrows and diamonds indicatethe effect on χ produced by replacing solvent 1 with solvent 2.

[0118] By implication any solvent can be “good” or “bad” as definedusing the Flory interaction parameter, χ. In terms of χ, these solvencyproperties are roughly χ<0.5 for good solvents and χ>0.5 for badsolvents, with the condition of χ=0.5 defining a “theta” solvent. Thegood-bad solvent transition is not a step change, but is instead agradual variation in the solubility of the polymer in the chosensolvent. Consequently, changing the solvent quality changes the affinityof the polymer with itself, either through intra-chain interactions ifthe solution is sufficiently dilute, or through inter-chaininteractions. The effects of changing solvent quality on polymersolubility can be seen in experimental data, or theoretically as shownin equation 1:

ν=(1−2χ)a ^(d)   (1)

[0119] where μ is excluded volume of a single chain, χ is the Floryinteraction parameter and a^(d) is the monomer volume. The theta point,where the chain is unperturbed, is therefore where μ=0 or χ=0.5. Forμ<0, phase separation occurs and there is an equilibrium between nearlypure solvent and a polymer-rich phase. However, due to the large size ofthe polymer molecules discussed herein, there is always some polymer inthe solvent phase, and similarly there is always an appreciable amountof solvent in the polymer rich phase. The solvent can be a singlechemical species, or a mixture of species that is not necessarilylimited to low molecular weight compounds.

[0120] It is usually considered that PVA phase separates through aspinodal decomposition process. Note that there is no rate dependenteffect in this equation, although the spinodal decomposition processdoes have a characteristic rate, and it is a balance of rate effectsthat is exploited in the embodiments of the present invention. For PVAin water, the Flory interaction parameter is weakly dependent onconcentration but χ>0.5 for polymer volume fractions greater than about5%. The PVA gel has a higher interaction parameter than the solvent butthere is no dependence of χ on molecular weight or crosslink density ofthe gel.

[0121] In preferred embodiments, χ of the second solvent must be morepositive than the χ of the first solvent (dissolved PVA solvent) and ispreferably be in the range of 0.25 to 2.0. Preferably χ of the firstsolvent is in the range of 0.0 to 0.5. In general, the temperatureduring processing may vary from just above the freezing point of the PVAsolution to the melting point of the physical crosslinks formed in theprocess. The preferable range is from about 0 degrees Celsius to about40 degrees Celsius. Note that χ is coupled to temperature andconcentration. In preferred embodiments the temporal and spatial changein X of the PVA solution (the first solvent) is controlled by contactwith another miscible solution (comprising the second solvent), whereinthe second solvent modifies the first.

[0122] The removal of the need for chemical crosslinkers and radiationprocessing allows a greater variety of embedded components. For instancemany bioactive materials are highly intolerant of chemical crosslinkersand radiation. In addition although in general the freeze-thaw processis gentle on bioactive components there can certainly be envisagedpolymers, biopolymers or cells that either cannot be frozen, or act asantifreeze hence preventing the freezing.

[0123] The method used to create these thetagels is likely to produce awider range of material properties and greater control over the physicalstructure of the final gel than is possible with competing cryogels. Theadvantage of thetagels over thermally cycled PVA gels for certainapplications is outlined below.

[0124] Cryogels have a fairly low resolution with regard to their finalproperties because each thermal cycle produces a dramatic change in thematerial properties of the gel. The thetagels produced demonstrate thatthe concentration of the solvent produces a monotonic decrease inswelling ratio once the theta value is passed (See FIG. 7). It isdesirable that the ultimate crosslink density is adjustable inproportion to the resolution achievable in the solvent concentration.For example, for the 10% PVA solution immersed in NaCl, the weightpercentage of PVA in the final gel varies at a rate of about 7%/moleNaCl.

[0125] The preferred embodiments of the present invention provide gelshaving a starting weight (based on the weight of the solution) fromabout 1 weight % to about 50 weight % PVA. In preferred embodiments, theweight percent ranges from about 12% to about 29% PVA in the final gelwhere the immersion solution used was about 2.0 about 3.0 M NaCl (aq).This range of final PVA weight percentages is comparable to that whichcan be achieved by thermally cycled PVA. With higher NaCl solutions (inexcess of 6.0 M) percent PVA of the final gel increases monotonicallywith NaCl concentration. It has been found that PVA thetagels can bemade that exhibit a smooth gradient in spatial properties. In contrast,gradients of properties cannot easily be manufactured in cryogels.Instead, the usual approach is to generate an array of stacked lamellaeindependently that must be joined in dissolved PVA and then cycledagain. Sharp differences in modulus in such an array would create amaterial with undesirable mechanical properties and with inhomogeneousinterfaces. In preferred embodiments, PVA thetagels having a smoothgradient in mechanical properties, can be used to make a prostheticintervertebral disk a central lower modulus “pulposus” having adequatecompressive strength and a higher modulus peripheral “annulus” thatminimizes creep and undesirable distortion.

[0126] Modulus enhancement can be accomplished by incorporation of ionicspecies. For thetagels produced in NaCl, it is possible to includenatural (hyaluronic acid) or synthetic (PAA) polymers to create gelswith strain variable compressive moduli. Gelling a PVA/PAA solution instrong NaCl shield the ionizable charges in the PAA while the PVA iscrosslinked around the collapsed PAA. Re-equilibration in deionizedwater will allow expansion of the PAA and pre-stress the PVA matrix. Theresulting construct should have a very different mechanical compressivemodulus due to the repulsion of the fixed charges on the incorporatedPAA.

[0127] It is known in the art that PVA elicits little or no hostbiological response when implanted in animals. For this reason, PVAs areused in a variety of biomedical applications including drug delivery,cell encapsulation, artificial tears, artificial vitreous humor, contactlenses, and more recently as nerve cuffs. However, PVA has generally notbeen considered for use as a load bearing biomaterial primarily becauseof its low modulus and poor wear characteristics. The loads that anyvertebral implant must withstand will be reasonably high (on the orderof 4 MPa in compression) requiring a high compressive modulus. In vivo,the compressive axial load on the intervertebral disk is transferred bythe nucleus pulposus to a tensile circumferential load in the annulusfibrosis. Any biomaterial intended to replace the function of anintervertebral disk in its entirety must incorporate similar anisotropicproperties.

[0128] To improve overall strength, PVA modulus and wear characteristicscan be enhanced by the formation of either chemical or physicalcross-links. Cross-linking PVA by the addition of chemical agents (suchas polyaldehydes), through irradiation, or by freeze-thaw cycling, hasbeen shown to improve the durability of PVA gels. However, chemicaladditives can leave unwanted residual reactive species behind that makethe final product unsuitable for transplant, while irradiation mayadversely affect any bioactive material encapsulated in the matrix.Thus, the generation of extensive physical cross-links throughfreeze-thaw cycling has substantially improved the durability of PVAwithout the negative side effects produced by chemical or irradiationinduced crosslinking. Recent investigations suggest that the physicalcrosslinks produced by freeze-thaw cycling might generate biomaterialswith moduli suitable for use as biocompatible replacements for loadbearing structures such as articular cartilage or intervertebral disk.

Solvation of Polymers and the “Theta” Point

[0129] Polymers in solution are complex molecules in perpetual dynamicmotion. The configuration of an ideal polymer chain is usually describedas a “random walk”, where the molecule is assumed for simplicity to befreely jointed and free to move where it will. This behavior results inthe polymer assuming a spherical shape with a Gaussian distribution. Inreality the chain has a number of forces acting on it to define itsshape and behavior. In free solution the chain is subject to randommotion from Brownian fluctuations arising out of the temperature of thesystem. At the same time there is a force arising out of how the chaininteracts with itself (since it is a long, extended molecule) and itssurroundings.

[0130] If the polymer is easily solvated by the solution (i.e., it is ina first solvent not having a χ value sufficient for gelation) it swellsas it tries to maximize the amount of polymer chain that is exposed tothe solvent. In the first solvent, the energy of interaction between apolymer element and a solvent molecule adjacent to it exceeds the meanof the energies of interaction between the polymer-polymer andsolvent-solvent pairs as described by Flory, P. J. in, Principles ofPolymer Chemistry, page 424, Cornell University Press, 1953, theteaching of which are herein incorporated by reference in theirentirety. The chain is now in a perturbed state and resists contact withneighboring chains and equally resists mechanical compression anddeformation. As the solvency changes, this swollen configurationcollapses as the quality of the solvent falls.

[0131] At the theta point, the solvent quality is such that the randomBrownian motions are enough to keep the chain in an ideal, Gaussiandistribution. Below this critical threshold the chain segments prefer tobe next to each rather than to a solvent molecule, and the chain shrinks(i.e. a second solvent having a χ value sufficient for gelation). TheFlory interaction parameter, χ is dimensionless, and depends ontemperature, pressure, etc. The first solvents have a low χ, while thesecond solvents have a high χ, with a transition at about χ=0.5. Thecase χ=0 corresponds to a solvent which is very similar to a monomer. Ina lattice model this is the case where the free energy comes entirelyfrom the entropy associated with various chain patterns on the lattice.In such a case, temperature has no effect on structure, and the solventis said to be “athermal.” Athermal solvents are a particularly simpleexample of good solvents. In most cases the parameter χ is positive asdescribed by de Gennes, P. G. in, Scaling Concepts in Polymer Physics,First ed. p. 72: Cornell University Press (1979). If the solvent qualityis poor enough, the chain will completely precipitate out of solution.This effect can also be obtained by manipulation of the temperature ofthe solution.

[0132] Once the concentration of the polymer solution is high enough,adjustment of the solvent quality can be achieved by replacing at leastpart of a first solvent with a second solvent that forces inter-chaininteraction as well as intra-chain interaction. Once the physicalcrosslinking has occurred, the later presence of a good solvent, whichnaturally swells the free polymer, is balanced by the physicalcrosslinking. With inter-chain associations the polymer chains are nowconstrained at certain pinning-points. Consequently as the polymer issolvated, and stretches, it becomes more deformed and is forced intotension. It is the competition between the solvation of the polymerchains and this tension in the deformed chains that give gels theirinteresting mechanical behaviors. In addition, under certain conditionsthe polymer chains can be ionized, consequently generating a charge.Adjacent like charges will result in further swelling due toelectrostatic repulsion. This is part of the mechanism that givesnatural cartilage (collagen and glycosaminoglycans) its high modulus,and high hygroscopic properties.

Gelation Mechanism in PVA

[0133] Freeze-thaw cycling of solutions of PVA polymer results in theformation of physical cross-links (i.e. weak bonding through an“association” of the polymer chains). PVA hydrogels formed in thismanner are termed “cryogels” and are described, for example, in U.S.Pat. Nos. 6,231,605 and 6,268,405, the teachings of which areincorporated herein by reference in their entirety. Importantly, thetechniques utilized to create PVA cryogels do not require theintroduction of chemical crosslinking agents or radiation. Cryogels aretherefore easily produced with low impact on incorporated bioactivemolecules. However, incorporated molecules are limited to those that cantolerate the freeze-thaw cycles required to make the gel. Thus theresulting material can contain bioactive components that will functionseparately following implantation. PVA cryogels are also highlybiocompatible (as will be the proposed PVA “thetagels” to be presentedlater). They exhibit very low toxicity (at least partially due to theirlow surface energy), contain few impurities and their water content canbe made commensurate to tissue at 80 to 90 wt %.

[0134] There is still some debate over the exact mechanism that drivesthe gelation of PVA through a freeze-thaw cycle. However, three modelshave been proposed to explain the physical crosslinking that occursduring the freeze-thaw cycle: 1) direct hydrogen bonding; 2) directcrystallite formation; and 3) liquid-liquid phase separation followed bya gelation mechanism. The first two steps suggest that the gel formsthrough a nucleation and growth (NG) phase separation, whereas the thirdoption pictures the process as a spinodal decomposition (SD) phaseseparation. Hydrogen bonding will form nodes and crystallite formationwill form larger polymer crystals. However both of these mechanisms willform closely connected crosslinks, with relatively small crosslinkingnodes. This observation is supported by studies on the gelationmechanism of PVA. Spinodal decomposition on the other hand causesredistribution of the polymer into polymer rich and polymer poor regionsfollowed by a gelation process which results in more distantly spacedcrosslinks. It is thought that phase separation through spinodaldecomposition is likely to be responsible for the improved mechanicalproperties of PVA after crosslinking and occurs due to a quenching ofthe polymer solution. During the freezing process, the system undergoesa spinodal decomposition whereby polymer rich and poor phases appearspontaneously in the homogeneous solution. This process occurs becausethe phase diagram of quenched PVA (and polymers in general) at certaintemperatures can have two coexisting concentration phases. The polymerrich phases are therefore highly concentrated which enhances the natural(weak) gelation of the PVA.

[0135] For cryogels, the physical characteristics depend on themolecular weight of the uncrosslinked polymer, the concentration of theaqueous solution, temperature and time of freezing and the number offreeze-thaw cycles. Thus the properties of a cryogel can be modulated.However, since the material's properties change dramatically at everyfreeze-thaw step, control over the properties of the finished gel issomewhat limited. The thetagels described broaden the range offunctionality currently provided by PVA cryogels.

[0136] In general, the modulus of the PVA cryogel increases with thenumber of freeze-thaw cycles. In one experimental series, thermallycycled PVA cryogels had compressive moduli in the range of 1-18 MPa andshear moduli in the range of 0.1-0.4 MPa. Stammen, J. A., et al.,Mechanical properties of a novel PVA hydrogel in shear and unconfinedcompression Biomaterials, 2001 22: p. 799-806.

[0137] As cryogels are crosslinked by physical and not chemical means,there is some concern about their structural stability. The modulus ofPVA in aqueous solution increases with soak time in distilled water atconstant temperature. In one experiment, conducted over 40 days, themodulus increased by 50%. Putatively, during aqueous aging, the increasein strength, with the concomitant loss of soluble PVA, is the result ofan increase in the order of the supramolecular packing of the polymerchains. There are significant implications in this data for thelong-term storage effects of the freeze-thaw gelled PVA.

[0138] It is also important to understand the effects of loss of polymerover time and how that impacts the local host biological environment. Itshould be noted that in this example, the cryogel was only freeze-thawcycled once, although others have shown PVA dissolution followingmultiple freeze-thaw cycles. In general, there is very littleinformation about the stability of PVA cryogel modulus under repeatedload cycling (fatigue).

[0139] As might be expected, the swelling of PVA cryogels at any timepoint decreases with increasing number of freeze-thaw cycles, indicatinga densification of the PVA gel, most likely due to a higher crosslinkdensity. In the long term, following gelation and under staticconditions, the ultimate swelling ratio decreases while the modulusincreases with time.

[0140] In freeze-thaw processing, temperature is used to force a phaseseparation of the PVA solution, thus enhancing the gelation mechanism inthe PVA (it should be noted that even at room temperature a solution ofPVA begins to gel weakly over time).

[0141] Solvent quality is related to both temperature and the chemicalinteraction of the solvent to the polymer, and is conveniently describedby the Flory interaction parameter χ. In a preferred embodiment, themanipulation of the solvent quality through some process other thantemperature allows much greater control over the gelation process whilepermitting the method to be practiced at approximately room temperature.In particular, by using aqueous based solvents for PVA, the system canbe chosen to minimize impact on materials embedded in the PVA, and canallow fine spatial and temporal control over the final structure of thegel. In a particular solvent, the critical parameter defining thetransition from the first to second solvent (and hence driving the phaseseparation) is known as the theta temperature. An example list ispresented in Table 1 below. TABLE 1 Theta temperatures for PVA invarious solvents (Brandrup, J. & Immergut, E. H. , Polymer Handbook,3Ed. 1989, NY, John Wiley & Sons). Solvent Volume Ratio ThetaTemperature [° C.] t-Butanol/Water 32:68 25 Ethanol/Water 41.5:58.5 25Methanol/Water 41.7:58.5 25 i-Propanol/Water 39.4:60.6 25n-Propanol/Water 35.1:64.9 25 NaCl/Water 2 Moles/L 25 Water — 97

[0142] Physically cross-linked PVA gels may also be produced throughthermal cycling (not necessarily with freezing) combined withdehydration. Such gels are potentially suitable for use in load bearingapplications (i.e. artificial articular cartilage).

[0143] Examination of the material properties of this thermally cycledPVA found that the material distributes stress more homogeneously thanstiff single-phase biomaterials (ultrahigh molecular weight polyethylene(UHMWPE)) and preserves the lubrication film gap readily in simulatedarticular cartilage loading. The material sustained and distributedpressure in the thin film of between 1 and 1.5 MPa. In transient loadtests, the PVA withstood and distributed loads of nearly 5 MPa.

[0144] Studies have been conducted that further examined the wearproperties of their thermally cycled, dehydrated PVA under a variety ofconditions. The wear rate found in unidirectional pin-on-disk (againstalumina) experiments was comparable to that of UHMWPE (although thistest is probably not the most suitable to perform for biologicalimplants). However, in reciprocating tests, the wear rate was up to 18times larger. To improve the wear properties, PVA of higher molecularweight and additionally cross-linked by gamma-radiation (doses over 50kGy) was tested. Such treatment reduced the wear rate considerably (toabout 7 times that of UHMWPE). However, in both radiation and thermallycrosslinked PVA the wear rate does not appear adequate for applicationswhere the opposing surface has high hardness. Additionally, irradiationwould adversely affect bioactive materials loaded into the gel.

[0145] Methods in accordance with a preferred embodiment include thefollowing:

[0146] PVA solutions. To make the 10% solution, 20 grams of PVA (100kg/mole; 99.3+% hydrolyzed; J T Baker) was dissolved in 180 grams ofdeionized water at 90° C. for one to two hours. To make the 20%solution, 30 grams of PVA was dissolved in 180 grams of deionized water,the solution was stirred continuously until 60 grams of water evaporatedto generate a final solution of 20% PVA.

[0147] PVA gelation. 4-5 ml of PVA solution of 10 or 20 weight percentwere injected into pre-wetted Slide-A-Lyzer Dialysis cassettes (Pierce,Rockford, Ill.) with a molecular weight cutoff of 3500 Daltons. The 10%PVA solutions were then immersed in NaCl aqueous solutions of 1.5 M, 2.0M, 2.5 M or 3 M. The 20% PVA solution was immersed in 3.0M NaCl. Todemonstrate that the gelation effect was not NaCl/aqueoussolvent-dependent, a 10% PVA solution in a dialyzer cassette wasimmersed in a 50/50 methanol/water solution. After 3 days, all of thecassettes containing 10% PVA solution were removed from their respectivesolvents. The gels were then removed from the cassettes and placed in DIwater for at least 5 days to allow initial PVA crystal dissolution thecassette containing the 20% PVA solution was removed after 3 days. ThePVA gel was removed from the cassette and a portion was stored in DIwater. The remaining PVA gel was returned to the 3M NaCl solution. At 6and 12 days, portions of the 20% PVA gel were removed and placed into DIwater for at least five days before further testing.

Quantitative Characterization

[0148] To quantify the effect on the structure of the gels of immersionsolution molarity and time immersed, differential scanning calorimetry(DSC), gravimetric swell ratio analysis and dynamic mechanical analysis(DMA) were performed on the samples.

[0149] Differential Scanning Calorimetry. DSC thermograms were obtainedusing an instrument, for example, a TA Instruments Q1000 (TAInstruments, New Castle, Del.). Selected wet PVA gel samples between 5and 15 mg were removed from deionized water storage after 5 days,blotted dry and crimped into alodized-aluminum hermetic pans. Scans wereperformed at 5° C./min from 5° C. to 120° C. The total enthalpy changefor the melting of the gel physical crosslinks was estimated using alinear integration from the departure from baseline (typically near 40°C.) to return to baseline (typically near 90° C.). Following DSCanalysis, the hermetic pans were punctured, weighed and placed in avacuum oven for dehydration. After two days of dehydration the pans werereweighed to determine the percent PVA in the original sample.

[0150] Gravimetric swell ratio. Samples of PVA gel from each sample wereremoved from deionized water storage after 5 days, blotted with a tissueand dehydrated in a vacuum oven for 2 days. The gravimetric swell ratiowas calculated as the ratio of the mass of water in the gel to the massof PVA in the gel.

[0151] Dynamic Mechanical Analysis. To examine the effect of curingsolvent quality, dynamic mechanical analysis was performed using aPerkin-Elmer TMA 7 (Perkin Elmer, N.J.) on the 10% PVA 3 M NaCl and 2 MNaCl samples. To examine the effect of aging in the curing solvent, DMAwas also performed on the 20% 3 M NaCl 3 day and 12 day samples. Sampleswere cut into rectangles and tested in unconfined compression with astatic load of 250 mN (10% samples) or 1000 mN (20% samples). Thestorage (and loss moduli for the 10% samples) were determined for afrequency sweep from 1 to 2 Hz at room temperature.

[0152] In a preferred embodiment, forcing poly(vinyl alcohol) polymerchains in solution into close proximity (through a spinodaldecomposition mechanism) results in the formation of a physicalassociation that is resistant to dissolution. This methodology generatesa PVA hydrogel employs the controlled use of the second solvents havinga χ value sufficient to cause gelation to force the PVA chains tophysically associate. It is critical that the solvent quality iscontrolled carefully, and in particular for larger components, that thesolvent “front” enters the PVA solution in a controlled manner.NaCl/deionized water and methanol/deionized water solutions atconcentrations in the neighborhood of their “theta” value for PVA wereused to force the physical association and subsequent gelling of thePVA. Gels formed in this way are termed “thetagels”.

[0153] The physical appearance of the hydrogel depends on the molarityof the solution into which the PVA solution is immersed. FIG. 3demonstrates the progression of the gelation of the PVA hydrogel duringexposure to NaCl solutions near the “theta” concentration. As exposuretime increases, the PVA solution becomes stiff and opaque for thesolutions at or above the theta concentration and temperature. Forsolutions appreciably below the theta concentration, little or nogelling is apparent. Immersion of the PVA solution into the 50/50water/methanol solution also resulted in the generation of a uniform PVAhydrogel.

[0154]FIG. 3 shows 10% PVA solution in dialyzer cassettes after 1 day(top) and 3 days (bottom) of immersion in curing solution. From left toright: 1.5 M NaCl, 2.0 M NaCl and 3.0 M NaCl. The 1.5 M solution doesnot gel the PVA, but the 2.0 M solution and 3.0 M solution do gel thePVA. Note the progressive opacification of the 2.0 M gel and theshrinkage of the 3 M gel from the edges of the cassette as the samplecompacts with time (indicated with arrow).

[0155]FIG. 4 demonstrates the difference between 10% PVA exposed to 3.0M and 2.0 M solutions for 3 days (after photographed subsequentequilibration in deionized water). PVA gels were generated by immersionin 3.0 M (left image of each pair) and 2.0 M (right image) NaClimmersion solution. Note that the gels are uniform and opaque. The gelexposed to 3.0 M NaCl swells less and is more compact followingequilibration in deionized water. The hydrogels that result are uniformand opaque. The PVA exposed to 2.0 M NaCl is more highly hydrated thanthat exposed to the 3.0 M NaCl. The increased swelling is an indicationthat the density of physical crosslinks is lower in the gel exposed tothe 2 M NaCl solution. Thus, gels formed in this way are “tunable” withrespect to mechanical properties. Further, gradient gels can be madeusing the method through manipulation of the spatial NaCl concentration.

[0156]FIG. 5 shows a hydrogel formed from a 10% PVA solution that wasexposed to a spatially varying NaCl concentration. Note, the variationin both the translucency of the gel and in the swelling ratio. Theopaque part of the gel was exposed to 3.0 M NaCl while the clear partwas exposed to a concentration below the theta concentration (2.0 M atroom temperature). The ability to generate a gradient is relevant to thegeneration of a total disk replacement nucleoplasty, with a rigid outerlayer (annulus fibrosis) and a softer center (nucleus pulposus).

Differential Scanning Calorimetry

[0157] For thermally cycled PVA gels, an endotherm between 30 degreesCelsius and 90 degrees Celsius represents the energy required to disruptthe physical crosslinks formed during the thermal processing. For PVAthetagels in accordance with a preferred embodiment, a similar endothermwas present. FIG. 6 compares the DSC thermogram of a thermally cycledPVA cryogel and a PVA hydrogel formed in 3.0M NaCl. The transitions havesimilar melting endotherms and occur at virtually the same temperature.

[0158] The enthalpy change of this endothermic transition gives a goodindication of the amount of crosslinking in the gel as a result of thesolution conditions. For a 10% PVA solution for example the enthalpychange obtained after immersion for 3 days in a 2.0M solution of NaClwas 16.9 J/g. In contrast the same initial PVA solution yielded anenthalpy change of 19.9 J/g after 3 days in a 3M solution. This resultindicates that solution concentration and soak time both positivelyimpact the amount of physical crosslinking in the gel.

Gravimetric Swell Ratio

[0159] In a preferred embodiment, increasing the molarity of NaCl in thesolvent increases the amount of PVA present in the hydrogel per unitmass. FIG. 7 shows the relationship between the percentage of PVA in thegels (fully equilibrated in deionized water) and the molarity of thesolution in which they were cured. Because the PVA is not rigidly heldin the dialysis cassette, it is free to expand or contract under theinfluence of the local forces on the PVA during the gelation process. Itis therefore possible for the final PVA concentration to exceed that ofthe initial PVA solution if the PVA gel has collapsed. FIG. 8 shows thegravimetric swelling ratio for PVA cured in solutions of varying NaClmolarity. For the 20% PVA solution, the 3 day value of swelling ratioand percentage of PVA matched that of the 10% PVA solution (not shown).After 12 days of immersion in 3 M NaCl (and 5 days of equilibration indeionized water), the 20% PVA solution formed a gel that was 29% PVA.

Dynamic Mechanical Analysis

[0160] In a preferred embodiment, the solution conditions and time ofaging have a marked effect on the visible structure of the gels, and ontheir thermal properties. Both effects suggest that there is likely tobe an influence on the mechanical properties as well. This suppositionis born out by qualitative examination of the samples, but for morerigorous analysis mechanical testing was performed on the samples usingDMA. FIGS. 9 and 10 present data taken from three of the samples. FIG. 9shows that the complex modulus of the sample increases with aging(keeping all other solution conditions constant). In fact, this increasein the modulus is also paralleled by a densification of the final PVAgel. FIG. 10 examines the change in modulus corresponding to the changein solution molarity (once again keeping all other parameters constant).In this FIG. 10, there is a clear indication that the storage (i.e.,elastic component) modulus rises sharply as the solution molarity isincreased (remember at room temperature 2 M NaCl is approximately atheta solvent) whereas the loss (i.e., damping) modulus is barelyaffected.

[0161] Cryogels have a fairly low resolution with regard to their finalproperties because each thermal cycle produces a dramatic change in thematerial properties of the gel. The thetagels produced demonstrate thatthe concentration of the solvent produces a monotonic decrease inswelling ratio once the “theta” value is passed (See FIG. 7). Thus, theultimate crosslink density can be fine tuned in proportion to theresolution achievable in the solvent concentration. For example, for the10% PVA solution immersed in NaCl, the weight percentage of PVA in thefinal gel varies at a rate of about 7% per mole NaCl.

[0162] In preferred embodiments, the PVA thetagels can be made thatexhibit a smooth gradient in spatial properties. Gradient propertiescannot easily be manufactured in cryogels. Instead, the usual approachis to generate an array of stacked lamellae independently that must bejoined in dissolved PVA and then cycled again. Sharp differences inmodulus in such an array create a material with undesirable mechanicalproperties and with inhomogeneous interfaces. A preferred embodimentincludes a composite annulus fibrosus/nucleus pulposus implant, thatbenefit from technology enabling a smooth gradient in mechanicalproperties, wherein a central lower modulus “pulposus” provides adequatecompressive strength and a higher modulus peripheral “annulus” minimizescreep and undesirable distortion.

Modulus Enhancement: Incorporation of Ionic Species

[0163] For thetagels produced in NaCl, it is possible to include natural(hyaluronic acid) or synthetic (PAA) polymers to create gels with strainvariable compressive moduli. Gelling a PVA/PAA solution in strong NaClwill shield the ionizable charges in the PAA while the PVA iscrosslinked around the collapsed PAA. Re-equilibration in deionizedwater will allow expansion of the PAA and pre-stress the PVA matrix. Theresulting construct has a very different mechanical compressive modulusdue to the repulsion of the fixed charges on the incorporated PAA.

Generating Gradient Thetagel

[0164] In another preferred embodiment, to make the gradient thetagel:10%, 20% and 30% solutions of 100kg/mole PVA are made as describedhereinbefore. A dialyzer cassette is split in half and each half bondedto one side of a 1×1×1 cm plexiglass box that is filled with the 10% PVAsolution. The sealed box is placed into a temperature controlled“Ussing” style chamber where it is subjected to a constant 4 molar NaClconcentration difference (see FIG. 10). After the number of days wherefurther changes in the gel are insignificant, the gradient gel isremoved from the chamber and placed in deionized water for five daysprior to further testing. Resulting gels are tested as describedhereinbefore.

[0165] In another embodiment, spatial gradient can be generated usingtemporal oscillations in concentration. The concentration in the chambercan be modulated temporally to provide a gel, having a softer interiorregion than the peripheral region where a higher crosslinking occurs.

[0166] The chamber 100 includes a cartridge 140 containing a gel 160.The chamber can be divided into sub-chambers or regions including twoimmersion solvents 112 and 122. In a preferred embodiment, the solventshave the same concentration. In another preferred embodiment, theimmersion solvents have different concentrations that cause a spatialgradient in the gel. Membranes 150, 154 are permeable membranes thatallow the immersion solvents to selectively flow into the vinyl polymersolution. Membrane 130 provides an impermeable barrier to the flow ofany solvent.

Dehydration

[0167] Preferred embodiments of the present invention are directed atcontrollably structuring gels. In a particular embodiment, in order topromote smooth dehydration and to homogenize the physical crosslinkingof the PVA thetagel, the gel or solution of PVA may be immersed in aseries of solutions, or in a bath of smoothly changing solvent quality,each with a higher Flory interaction parameter than the previoussolution. This prevents the local “crashing out” of PVA at the surfacedirectly in contact with the immersion solution. The term “crashing out”as used herein is associated with a phenomenon akin to precipitationbecause of the Flory interaction parameter. The polymer chains prefer toassociate with themselves instead of the solvent as the Floryinteraction parameter is above the theta point and thus precipitate orcrash out.

[0168] In one preferred embodiment, a thetagel may be created by firstimmersing the contained PVA solution into a solvent which has a Floryinteraction parameter that is higher than the theta point for the PVAsolvent pair. After a period of time the contained PVA is immersed inanother solvent, which has a Flory interaction parameter lower than thetheta point for the PVA solvent pair. The process can continue withimmersion of the contained PVA in solutions having successive decreasesin the Flory interaction parameter until the desired interactionparameter value for the final gel is reached.

[0169] A method to form a thetagel in accordance with a preferredembodiment of the present invention includes immersing contained 5-20%PVA in DI, followed by immersion for a range of 1 hour to 1 day in 2.0 MNaCl, followed by immersion for a time period ranging between 1 hour to1 day in 3.0 M NaCl, followed by immersion for a time period of 1 hourto 1 day in 4.0 M NaCl, and followed by immersion for a time periodranging from 1 hour to 1 day in 5.0 M NaCl.

[0170] In another preferred embodiment, the PVA solution may besubjected to a gradually changing solvent quality through a similarrange of electrolyte concentrations by the gradual addition of aconcentrated NaCl solution to a DI water bath such that the change ofthe salt concentration is slower, or equal to, the diffusion processinto the gel.

[0171] A method in accordance with a preferred embodiment includesimmersing contained 5-20% PVA in 1 liter of 1.5 M NaCl, and adding 6 MNaCl at a rate of 0.5 ml per minute to raise the electrolyteconcentration at a rate of 0.0038 N/min and reaching 5 M NaCl afterapproximately 12 hours.

[0172] In another embodiment, the PVA solution may be subjected to oneor many freeze-thaw cycles to fix the gel into a particular shape. Itmay then be immersed in a series of solutions having successively higherFlory interaction parameters until the final desired Flory parameter isreached.

[0173] A method in accordance with a preferred embodiment includesdissolving 5-20% PVA in DI, subjecting the solution to freeze-thawcycles (approximately 1-8 cycles), and subsequently for a period rangingbetween 1 hour to 1 day, immersing the resultant gel in 2.0 M NaCl. Themethod further includes immersing the PVA gel for a time period of 1hour to 1 day in 3.0 M NaCl, followed by immersion for a time periodranging from 1 hour to 1 day in 4.0 M NaCl and subsequently immersingfor a time period of 1 hour to 1 day in 5.0 M NaCl.

[0174] In an alternate preferred embodiment, a method to form a gelincludes dissolving a 5-20% PVA in DI, adding NaCl to the PVA solutionto generate a concentration from 0.01 to 2 M NaCl in the PVA solutionand then subjecting the PVA/NaCl solution to between 1 to 8 freeze-thawcycles.

Nanostructuring

[0175] Polyvinyl alcohol gel is an extremely biocompatible material thatcan be made reasonably stiff without the use of chemical crosslinking orirradiation. However, the material properties of the PVA do not matchthe requirements of materials for use in load bearing applications suchas, for example, artificial articular cartilage or intervertebral disks.A nanostructural enhancement of polymer systems in accordance with apreferred embodiment of the present invention indicates that PVA gels,which are already nearly suitable for use in load bearing orthopedicdevices, may become viable candidates for such applications.

[0176] Nanostructuring polyvinyl alcohol theta and hydrogels—particles.The addition of particles to polymeric materials can improve themechanical and thermal properties of the resulting material whencompared to formulations of the neat polymer. Recently, it has beenshown that the addition of nanoparticles to polymers can generatesimilar enhancements in the material properties, but with much lowerparticulate concentrations than those required of micron sizedparticles. This is particularly true when the material properties aredependent on surface area. In accordance with preferred embodiments, tostrengthen polyvinyl alcohol thetagels or hydrogels, the dispersion ofuncharged nanoscale particles or charged nanoscale particles withuniform or spatially varying surface charges into the solution prior togelation enhances the mechanical and thermal properties of the finalgel. Nanoscale particles, if dispersed properly, provide regularnucleation sites for physical crosslinking by adsorbing PVA chains totheir surfaces in accordance with a preferred embodiment of the presentinvention. As in rubber toughened plastics, these nanoparticles also actas stress concentrators, thus toughening the gel. Nanoscale particlesthat may enhance the properties of PVA gels are, for example, clays (forexample, but not limited to, Laponite, montmorillonite), fumed silica,titanium dioxide or hydroxyapatite. Surface treatments andmodifications, such as end grafting of polymers also adjust the way inwhich the particles interact with the polymer gel matrix in accordancewith a preferred embodiment of the present invention. These particlesmay also be biologically active, such as, for example, capable ofreleasing drugs to promote growth, or reduce inflammation.Nanostructuring is not limited to thetagels in accordance with apreferred embodiment of the present invention. However, the thetagels inaccordance with the present invention allow the formation of physicalcrosslinks around charged particles under solution conditions where thedebye length is reduced compared to the working solution. Thus when thegel is replaced in the working solution of lower electrolyteconcentration the particles interact through electrostatic forces andadd compressive strength to PVA thetagels as compared to PVA freeze-thawgels.

[0177] In one embodiment, nanoparticles are dispersed into solutions ofPVA. The solvent may be water, DMSO, methanol or any other solution thatexhibits a Flory interaction parameter that is lower than the thetapoint for the PVA solvent pair during solution preparation. ThePVA/nanoparticle mixture is then subjected to at least one freeze-thawcycle. Subsequent to the freeze-thaw cycling, the gelled PVA is immersedin a solvent that has a Flory interaction parameter near or higher thanthe theta point for the PVA/solvent pair to induce further physicalcrosslinking of the PVA/nanoparticle mixture.

[0178] A method in accordance with a preferred embodiment of the presentinvention includes mixing 5-20% PVA in DI with 1-10% fumed silica,freeze-thawing (1-8 cycles) the solution, followed by immersion for atime period ranging from 1 hour to 5 days in 2-5 M NaCl.

[0179] In another embodiment, the PVA/nanoparticle mixture is gelled byimmersion into a solvent that has a Flory interaction parameter near orhigher than the theta point for the PVA/solvent pair to induce physicalcrosslinking of the PVA/nanoparticle mixture. No freeze-thaw cycling isnecessary in this embodiment.

[0180] A method in accordance with a preferred embodiment of the presentinvention includes mixing 5-20% PVA in DI with 1-10% fumed silica,followed by immersion for a time period ranging from 1 hour to 5 days in2-5 M NaCl.

[0181] In another embodiment, the composite gels resulting from the twoexamples described hereinbefore are subject to further freeze-thawcycles.

[0182] In another embodiment, PVA solutions or gels containingnanoparticles are subject to the dehydration protocol as describedhereinbefore. A method in accordance with a preferred embodiment of thepresent invention includes mixing 5-20% PVA in DI with 1-10% fumedsilica, subjecting the solution for 1-8 cycles of freeze-thawing,followed by immersion for a time period ranging from 1 hour to 1 day in2.0 M NaCl, followed by immersion for a time period ranging from 1 hourto 1 day in 3.0 M NaCl, followed by immersion for a time period rangingfrom 1 hour to 1 day in 4.0 M NaCl and subsequently followed byimmersion for a time period ranging from 1 hour to 1 day in 5.0 M NaCl.

[0183] Nanostructuring polyvinyl alcohol thetagels andcryogels—functionalized molecular additives. The addition of particlesto the PVA solution prior to gelation can provide enhancement of thethermal and mechanical properties of the gel. However, there is a classof molecular additives that can be functionalized to promote physicalcrosslinking and can simultaneously act as stress concentrators.Polyhedral oligomeric silsesquioxane (POSS) can enhance mechanicalproperties of polymeric materials. Since the POSS molecules can befunctionalized, they can be tuned to associate with the PVA chains toenhance interchain crosslinking and to act as stress concentrators.Their extremely small size and large number of functionalized groups hasthe potential to provide better results than nanoparticle seeding.

[0184] All of the methods for generating thetagels or cryogels describedhereinbefore may be applied to solutions containing dispersedfunctionalized POSS molecules. In one preferred embodiment, POSSfunctionalized to display negatively charged oxygen groups can be usedto promote hydrogen bonding. The functionalized POSS is dispersed intoaqueous PVA solution and subjected to theta or freeze-thaw gelation(ranges 0.01 mM to 1 M OctaTMA POSS (tetramethyl ammonium salt) and5-20% PVA in solution).

[0185] In another preferred embodiment, POSS functionalized to displayalcohol groups is dispersed into PVA and subjected to theta orfreeze-thaw gelation (ranges 0.01 mM to 1 MOctahydroxypropyldimethylsilyl POSS and 5-20% PVA in solution)

[0186] In another embodiment, POSS functionalized to display at leastone PVA chain and at least one carboxyl or sulfate group can be used toproduce an extremely hydrophilic, tough artificial cartilage. Thepreferred POSS construct has at least one PVA chain at opposite cornersof the POSS with the 6 remaining functional groups expressing sulfate orcarboxyl groups. This structure can be “stitched” into the PVA gelnetwork via the thetagel process or freeze-thawing to produce anartificial cartilage with tunable properties.

[0187]FIG. 12 illustrates a quick freeze deep etch (QFDE) image of PVAgel structure in accordance with a preferred embodiment of the presentinvention wherein the PVA gel is formed by immersion in 5 M NaCl for 3days. The bar represents 100 nm. QFDE preserves the gel structure in itshydrated state.

[0188]FIGS. 13A and 13B are a cross-sectional and a close-up view of thecross-section of a PVA gradient hydrogel, respectively, prepared byfilling Plexiglass tubing with 10% PVA solution, performing one freezethaw cycle (8 hours at −21° C.; 4 hours at room temperature) thenimmersing in 3 M NaCl bath for at least 3 days, and subsequentlydehydrating in air for 60 hours and returning to deionized (DI) water inaccordance with a preferred embodiment of the present invention. FIGS.13A and 13B illustrate the presence of radial gradients in PVA inducedby air dehydration.

[0189]FIG. 14 illustrates a cross-sectional view of a PVA gradienthydrogel prepared by filling dialysis cartridge with 10% PVA solution,then immersing in a chamber having 3 M NaCl on one side and 6 M NaCl onthe other side for 3 days in accordance with a preferred embodiment ofthe present invention. FIG. 15 illustrates a close-up view of thecross-section of the PVA gradient hydrogel of FIG. 14 on the 6 M NaClside prepared by filling the dialysis cartridge with 10% PVA solution,and then immersing in a chamber with 3 M NaCl on one side and 6 M NaClon the other for 3 days. FIGS. 14 and 15 illustrate the presence oflinear gradients in PVA induced by static NaCl solution gradient.

[0190]FIGS. 16-19 illustrate nanostructured PVA gels in accordance withpreferred embodiments of the present invention. More particularly, FIG.16 illustrates a nanostructured PVA hydrogel prepared by mixing asolution of 10% PVA and 2 weight percent Laponite clay, subjecting to a1 freeze-thaw cycle, then exposing the solution to a 4 M NaCl solutionfor at least 3 days in accordance with a preferred embodiment of thepresent invention.

[0191]FIG. 17 illustrates a nanostructured PVA hydrogel prepared bymixing a solution of 10% PVA and 4 weight percent of silica, titratingto pH=3, subjecting to a 1 freeze-thaw cycle, then exposing to a 4 MNaCl solution for at least 3 days in accordance with a preferredembodiment of the present invention.

[0192]FIG. 18 illustrates a nanostructured PVA hydrogel prepared bymixing a solution of 10% PVA and 4 weight percent silica, titrating topH=10, subjecting to 1 freeze-thaw cycle, then exposing to 4 M NaClsolution for at least 3 days in accordance with a preferred embodimentof the present invention.

[0193]FIG. 19 illustrates a nanostructured PVA hydrogel prepared bymixing a solution of 10% PVA and 0.001 M octaTMA PQSS in water, thensubjecting to 1 freeze-thaw cycle in accordance with a preferredembodiment of the present invention.

[0194]FIG. 20 graphically illustrates storage moduli for hybrid andcontrol PVA gels in accordance with preferred embodiments of the presentinvention. The graphs are results of DMA testing on 4% silica/PVAnanostructured gel having a pH=10 (sample number 1), 4% silica/PVAnanostructured gel having a pH=3 (sample number 2), 2% Laponite/PVAnanostructured gel (sample number 3), 0.001 M, 10% PVA+octaTMA POSS(sample number 4), and a control gel (sample number 5). All gels weresubjected to 1 freeze-thaw cycle and then immersed in 3 M NaCl for threedays. Prior to DMA testing the samples were equilibrated in DI water forat least 24 hours.

[0195] The embodiments of the present invention provide methods for thecontrolled manipulation of the Flory interaction parameter in a solutionof vinyl polymer, in particular polyvinyl alcohol, to yield a workablefluid that gels in a controlled manner. The control of the solventcondition allows control of the gelation rate, which results in a timeperiod in which the PVA solution has only partially gelled, thuspermitting manipulation or working of the pre-cursor gel prior to finalgelation. During this time period the PVA solution is substantiallyfluid and can be injected, pumped, molded, or undergo any othermanipulative processing step. The final properties of the hydrogel,which include, but are not limited to, percentage crystallinity, crystalsize, free volume, and mechanical properties, are influenced by theinitial vinyl polymer concentration, the gellant concentration (i.e.,the final solvent quality) in the final mixture, the processingtemperature, and the mixing procedure.

[0196] One preferred embodiment of the method is shown in FIG. 21A. FIG.21A illustrates a flow chart of a method 400 of forming a PVA hydrogelincluding the step 402 of mixing a vinyl polymer and a first solvent;mixing a gellant in the vinyl polymer solution per step 404; andpreventing gelation of the vinyl polymer physical associations byheating the mixture to a temperature above the melting point of thevinyl polymer per step 406. More particularly, this temperature is abovethe melting point of any physical associations formed in the vinylpolymer. In an alternate embodiment, step 406 precedes step 404. Themethod 400 further includes the step 408 of inducing gelation of thevinyl polymer solution; forming a transiently workable viscoelasticsolution per step 410; controlling or modulating the gelation rate ofthe viscoelastic solution per step 412, for example, by modulating atleast one of the temperature, the pressure and the concentrationgradient; and forming the hydrogel per step 414 in accordance with apreferred embodiment of the present invention.

[0197]FIG. 21B illustrates a flow chart of methods of forming andproviding a vinyl polymer hydrogel in accordance with preferredembodiments of the present invention. These methods are directed atmanufacturing vinyl polymer based hydrogels by modulating solventquality. The methods include the step 424 of dissolving vinyl polymer inwater at, for example, greater than 80° C. at any desired concentration.The next step includes the preparation of a gellant as a powder per step426, or as a solution per step 430. The gellant can naturally be aliquid per step 428. The next step 432 includes providing a gellant insufficient concentration to be near (above or below) the critical thetacondition of a subsequent mixture when added to the vinyl polymersolution. The method then includes the step 434 wherein the gellant andvinyl solution are kept separately, the step 436 of loading thecomponents into a two or more chambered device such as a syringe or apump, and the step 438 of injecting the polymer solution into a regionof interest, such as a cavity in the body through a mixing apparatus.The solution arrives in the region of interest substantially mixed perstep 410.

[0198] The method 400 after step 432 can alternatively include the step442 of adding the gellant while mixing the vinyl solution, the step 444wherein the solution is mixed until it is homogenous and step 446wherein the vinyl polymer solution is still in a fluid and workablestate. The method 400 can include the step 448 of loading the polymersolution into a syringe or pump followed by step 450 of injecting thesolution into the region of interest in the body before the solution hassubstantially crystallized. In the alternative, after step 446, themethod 400 can include the step 452 of placing the polymer solution in amold shaped for a specific use or the step 454 of blow molding thepolymer solution to form a thin hydrogel membrane or per step 456 ofloading the polymer solution into a syringe or pump. Step 458 follows byinjecting the solution into the region of interest and per step 460triggering the gellant to drop solvent quality. Step 462 can follow theprocessing steps 440, 450, 452, 454 or 460 alternatively and includesthe permanent crystallization of the gellant that occurs substantiallyafter the above listed processing steps.

[0199] In several embodiments, the vinyl polymer is highly hydrolyzedpolyvinyl alcohol of about 50 kg/mol to about 300 kg/mol molecularweight. The vinyl polymer solution is about 1 weight percent (wt %) toabout 50 wt % of polyvinyl alcohol based on the weight of the solution.In preferred embodiments, the vinyl polymer solution is about 10 weightpercent to about 20 weight percent solution of polyvinyl alcohol basedon the weight of the solution.

[0200] The first solvent is selected from a group of solvents having alow χ value that is not sufficient to enable gelation. In severalpreferred embodiments, the first solvent is selected from the groupincluding, but not limited to, of deionized water, dimethyl sulfoxide, aC₁ to C₆ alcohol and mixtures thereof.

[0201] The second solvent, the gellant, is selected from a group ofsolvents having the property that raises the χ value of the resultantmixture of gellant and vinyl solution to χ>0.5 at a specifiedtemperature. In several embodiments, the gellant is selected from thegroup including, but not limited to, for example, alkali salts,glycosaminoglycans, proteoglycans, oligomeric length hydrocarbons suchas polyethylene glycol, enzyme-cleavable biopolymers, UV-cleavablepolymers, chondroitin sulfate, starch, dermatan sulfate, keratansulfate, hyaluronic acid, heparin, heparin sulfate, biglycan, syndecan,keratocan, decorin, aggrecan, perlecan, fibromodulin, versican,neurocan, brevican, a phototriggerable diplasmalogen liposome, aminoacids such as, for example, serine or glycine, glycerol, sugars orcollagen. The gellant can be added in the form of a solid or as anaqueous solution.

[0202] In one preferred embodiment, the gellant is added by being mixedwith a solution of the vinyl polymer held at an elevated temperature,preferably above the melting point of the vinyl polymer. The meltingpoint can be suitably determined by differential scanning calorimetry(DSC). With reference to the dashed curve illustrated in FIG. 6, forPVA, the elevated temperature is at least greater than 57 degreesCelsius, preferably greater than 80 degrees Celsius, more preferablygreater than 90 degrees Celsius. In general, with reference to agraphical representation of differential scanning calorimetry resultssuch as FIG. 6, the most preferred elevated temperature is in the linearportion of the curve at temperatures above the melting point.

[0203] The resultant mixture begins to undergo a spinodal decompositionas it mixes and cools. The mixture is injected into a body cavity,whereupon it forms a load-bearing gel over a period of time.

[0204] In one preferred embodiment the solvent quality of the entiremixture of PVA, water and secondary or tertiary components has a Floryinteraction parameter of 0.25<χ<0.8 and preferably in the range of0.3<χ<0.5.

[0205] The preferred embodiments of the invention provide an injectablehydrogel that can be used for orthopedic therapies, including nucleuspulposus augmentation or replacement, and augmentation of load bearingsurfaces in an articulated joint such as, for example, knee or hip. Inthe case of knee or hip augmentation, the injectable hydrogel can beused in early interventional therapy for those patients who, although inpain due to partial loss of articular cartilage, are not candidates fortotal knee or hip replacement.

[0206] The injectable hydrogel can also be used for non-load bearingapplications for replacement, repair, or enhancement of tissue. It canalso be used topically as a protective coating for burns or wounds. Thepreferred embodiments of the invention are especially suited tominimally invasive applications where small access holes in tissue arerequired. The access holes can have diameters of approximately 1-10 mmand can be located, for example, without limitation, in the annulusfibrosis, bone tissue, cartilage, or other tissues.

[0207] In another preferred embodiment, the gellant is added to a mixingsolution of the vinyl polymer held at an elevated temperature. Theresultant mixture begins to undergo a spinodal decomposition as it mixesand cools. This mixture can then be used to generate a hydrogel deviceto be manufactured using conventional processing means such as injectionor compression molding, blow molding, calendaring, or any other suitableprocessing step. This device can then be implanted as a load bearingdevice, or some other non-load bearing device such as a nerve cuff or aspart of a drug release system. This device can also be used innon-biological applications as protective hydrogel films, or sealingagents.

[0208] In another embodiment, the vinyl solution and gellant are notpre-mixed, but are co-injected through a tube via a mixing chamberhaving a tortuous path that facilitates mixing. The pre-cursor hydrogelcan be injected using a suitable dispenser directly into the targetlocation. FIG. 28A illustrates a flow chart of method 600 of forming aPVA hydrogel including the steps of providing a vinyl polymer and afirst solvent in a chamber of a first dispenser per step 602; placing agellant in a chamber of a second dispenser per step 604; mixing thevinyl polymer solution and the gellant in a mixing chamber per step 606and heating the mixture to a temperature above the melting point of thevinyl polymer 608. The method then includes dispensing the mixture perstep 610; delivering the mixture into a region of interest, such as acavity, per step 612; and inducing gelation of the mixture per step 614to form a PVA hydrogel in accordance with a preferred embodiment of thepresent invention. In certain embodiments, the step of providing a vinylpolymer and a first solvent includes the step of mixing the vinylpolymer and the first solvent. Similarly, in certain embodiments, thestep of placing the gellant in a chamber of the second dispenser chamberincludes the step of mixing the gellant and a second solvent.

[0209]FIG. 28B illustrates a flow chart of a method 620 of forming a PVAhydrogel in accordance with an alternate preferred embodiment. Thismethod 620 is similar to the method 600 illustrated with respect to FIG.28A however, the step of heating the polymer solution in a chamber of afirst dispenser to a temperature above the melting point of physicalassociations in the polymer (step 626) precedes the step of mixing thevinyl polymer solution and the gellant in the mixing chamber (per step628).

[0210] The step of inducing gelation as discussed with respect to theflow charts in FIGS. 28A and 28B includes the modution of temperation,in particular to lowering the temperature of the mixture of the vinylpolymer solution and the gellant below the crystallization temperature(melting point of the physical association). In alternate preferredembodiments the step of inducing gelation includes the release of activegellants from an inactive gellant complex. The inactive gellant complexincludes, without limitation, for example, enzyme cleavable polymers,heat denaturable polymers, thermal/chemical/photo/triggered liposomes orhydrogels; thermally triggered irreversible crystalline materials suchas starches, and degradable polymers.

[0211]FIG. 28C illustrates a flow chart of a method 650 for forming avinyl polymer hydrogel in accordance with a preferred embodiment of thepresent invention. The method 650 includes mixing a vinyl polymer andfirst solvent per step 652. The method 650 includes the step 654 ofpouring the solution into one barrel of a multi-barrel syringe, pouringa gellant into another barrel of the multi-barrel syringe per step 656,raising the temperature of the barrel above the melting point of thephysical associations of the vinyl polymer per step 658, centrifuge thebarrel per step 660, and injecting through a static mixture such as acannula having a tortuous path the mixture into a region of interest,for example, into a joint space per step 662.

[0212] The method 650 includes in an alternate embodiment the step of666 of mixing a gellant into a vinyl polymer solution, pouring thesolution into a single barrel per step 668, raising the temperature ofthe barrel above the melting point of the vinyl polymer physicalassociations per step 670, centrifuging the barrel per step 672, andinjecting the solution through a cannula or a syringe needle into theregion of interest per step 674. The steps of the method 650 can thusprovide material such as nucleus pulposis augmentation for a disksystem. This gel in accordance with the preferred embodiments conformsto the joint space or any region of interest.

[0213] Further, the method 650 includes for the different embodimentsthe following steps of inserting a closure device “for a diskaugmentation per step 664 or injecting concentrated gellant into anopening in the joint space to locally enhance the gelation rate andfinal mechanical properties per step 676 or per step 678 requiring nofurther procedure post the injection of the gellant into the region ofinterest per steps 662, 674. The latter steps 664, 676, provide for theaugmentation of, for example, the annulus fibrosis in a disk system.

[0214]FIGS. 29A-29F schematically illustrate a method 700 for formingand dispensing a vinyl polymer hydrogel in accordance with a preferredembodiment of the present invention. The method, includes per FIG. 29A,a system being supplied in three aseptic containers or two containerswherein the contents of the containers A and B are combined. The asepticcartridges contain PVA, solvent 1 (water) and gellant, respectively, asillustrated in FIG. 29B. The two components are mixed in a sealedcontainer at a temperature above 80° C. The components are thentransferred to a single barreled holder 704 as illustrated in FIG. 29Cand centrifuged whilst maintaining temperature to remove bubbles perFIG. 29D. As illustrated in FIG. 29E, a nozzle or syringe needle isattached to the single barreled holder which is then assembled onto aplunger system that can be mechanically or electrically actuated(ratcheted) to deliver the mix solution. As described herein before, themixed solution flows for a short period of time before becomingunworkable.

[0215]FIGS. 30A-30F schematically illustrate a method for forming anddispensing a vinyl polymer hydrogel in accordance with an alternatepreferred embodiment of the present invention. As illustrated in FIG.30A, the hydrogel system is supplied in three aseptic containers or twocontainers wherein the contents of containers A and B are combined. Theaseptic cartridges contain PVA, solvent 1 and gellant, respectively. Asillustrated in FIG. 30B, the PVA solution is mixed and heated and thegellant is also heated in container C. The ingredients are then loadedinto separate chambers of a twin barrel system while maintaining theelevated temperature as shown in FIG. 30C. The barrel system is thencentrifuged and/or vacuum degassed and injected through a static mixernozzle. As shown in FIG. 30E, a nozzle or syringe needle is attached andassembled onto a plunger system that can be mechanically or electricallyactuated (ratcheted) to deliver mix solution. The mixed solution flowsfor a short period of time before becoming unworkable.

[0216]FIGS. 31 A-31E schematically illustrate an alternative preferredmethod for forming and dispensing a vinyl polymer hydrogel in accordancewith an embodiment of the present invention. This embodiment includes ahydrogel system being supplied in a single cartridge. The aseptic doublebarreled cartridge illustrated in FIG. 31A contains PVA and solvent 1 inone and gellant in the other. The cartridge is heated to remelt the PVAin solution as shown in FIG. 31B. The system is then centrifuged and/orvacuum degassed and injected through a static mixer nozzle as shown inFIG. 31C. As illustrated in FIG. 31D, the nozzle or syringe needle isattached and assembled onto a plunger system that can be mechanically orelectrically actuated (ratcheted) to deliver the mixed solution. Themixed solution flows for a short period of time before becomingunworkable.

[0217]FIG. 32 schematically illustrates a dispenser 800 for providing avinyl polymer hydrogel in accordance with a preferred embodiment of thepresent invention. The dispenser includes a first chamber 810; a secondchamber 812; a first chamber piston rod 814; a second chamber piston rod816; a housing 818; a movable lever 820; a fixed handle 822; a mixingchamber 830; a mixing chamber fitting 832; a dispensing tube 834; adispensing tube fitting 836; a dispensing tube opening 838; atemperature controller 850; a connector 852; a mixing chamberheater/cooler 854; a chamber heater/cooler 856; and a dispensed mixture860. In preferred embodiments, vinyl polymer solution and gellant areprovided as premixed sterile solutions, preferably pre-packaged in firstchamber 810 and second chamber 812, respectively. A heater/cooler 854and 856 can include resistive heating, inductive heating, water jacketor Peltier effect heating/cooling elements. The temperature controller850 can be integral with the dispenser 800, or a separate unit,connected by connector 852. The entire dispenser 800 can be sterile,preloaded and intended for a single use.

[0218] In another preferred embodiment, drugs can be mixed with eitherthe vinyl polymer solution or the gellant so that the resultant injectedgel contains an encapsulated drug that can release over time.

[0219] In yet another preferred embodiment, a small amount of freeradical scavenger is added to the vinyl polymer solution in aconcentration of approximately 1 to 1000 parts per million. The freeradical scavenger can be any common free radical scavenger known tothose skilled in the art, but can include Vitamin E and hydroquinones.The purpose of the free radical scavenger is to minimize the effects ofionizing radiation, either gamma or electron beam (e-beam), which may beused to sterilize the material, prior to use. Radiation can eithercrosslink or cause scissioning in PVA solutions depending on theconcentration of the solution.

[0220] In another embodiment, the final mechanical properties of thehydrogel can be tailored by varying the initial starting concentrationof the vinyl polymer solution, and the concentration of the gellant inthe final mixture.

[0221] In another series of embodiments, the generation of conditionsconducive to force the gelation of the PVA entail the internal releaseof active ingredients or sequestered materials which can comprise anycombination of or, single gellant listed herein before. To change thetheta-point of the solvent relative to the PVA, or to alter theco-nonsolvency of the solvent relative to PVA, there exist potentialmethods based on the sequestration of “active” molecular species. Suchmechanisms serve to rapidly release the sequestered active molecularspecies to achieve local changes in solvency. If enough of thesequestering moieties are distributed through the system, it is possibleto effect a global change in solvency. Such a method would serve toalleviate precipitation problems associated with mixing the PVA with aparticularly active gellant. Suitable sequestration systems areavailable, including liposome sequestration, polymer sequestration,crystalline sequestration, gel encapsulation and degradableencapsulation In a preferred embodiment, liposome sequestration useslipid vesicles to separate their contents from the external environment.This system has been used successfully to induce rapid gelation ofpolysaccharide and protein hydrogels. Lipid vesicles can be induced torelease their contents by either thermal or phototriggering methods. Inpreferred embodiments, the gelation of a PVA solution prepared accordingto the present invention can be accelerated following application of asuitable trigger. A suitable trigger can be the gel/liposome compositionheated to body temperature. In a preferred embodiment of the presentinvention, an aqueous PVA solution is mixed with a suspension ofthermally triggerable liposomes containing a concentrated NaCl solutionor solid NaCl at a temperature below that necessary to induce release ofthe NaCl. Upon injection into a region of interest such as a body cavityat or near 37° C., the liposomes release the contained NaCl, changingthe Flory parameter of the solution and causing gelation of the PVA. Inother embodiments, other suitable gellants can be sequestered in thelipid vesicles to influence the gelation rate of PVA.

[0222] In another preferred embodiment, the sequestration system isbased on the increase of colligative activity of a polymer by cleavageof the polymer into multiple smaller fragments. In some complex polymersystems, degradation produces fragments that are more soluble than theoriginal molecule, for example, collagen. Such an increase in smaller,more active components shifts the solvency of the overall solution toinduce the gelation of PVA. Some examples of suitable polymericsequestration systems (and their enzymatic cleaving complements) arelisted, without limitation, in the table below. The table is notinclusive however the polymer sequestration concepts are intended toinclude all polymers, in particular, bio-compatible polymers orbiopolymers, and their particular polymer degradation mechanisms. In afurther embodiment, the two above approaches can be combined, usingtriggered liposomes to sequester the appropriate enzymes to producetriggerable cleavage of the gellant.

[0223] In one embodiment, fully formed purified type I collagen fibrilscan be mixed with PVA solution at temperatures below the denaturationtemperature of the collagen. The solution can be heated to induce thedenaturation of the collagen fibrils which would release soluble gelatinmolecules into the PVA, changing the relative solubility of the PVA andpotentially inducing PVA gelation. TABLE 2 Specific Cleavage Methods forSelected Gellants Degradation Degradation specifics Polymer mechanism Orenzyme complement Collagen Type I Heat denaturation Temperature 40-90°C. Collagen Type I Enzyme cleavage Collagenase (MMP I) All remainingcollagens Heat denaturation Temperature 40-90° C. All remainingcollagens Enzyme cleavage All MMP complements (MMPs 3-20) HyaluronicAcid Enzyme cleavage Testicular Hyaluronidase Hyaluron lyase ChondroitinSulfate Enzyme cleavage Chondroitinase family Heparin Enzyme cleavageHeparinase I/III All remaining Enzyme cleavage GAG enzyme complementsglycosaminoglycans Aggrecan Enzyme cleavage Aggrecanase All remainingEnzyme cleavage PG enzyme complements Proteoglycans

[0224] In another preferred embodiment, the sequestration method entailsthe confinement of active moieties in crystals that can be meltedirreversibly, producing a large change in the activity of thecrystalline component. In one preferred embodiment the crystallinecomponent is a starch, comprising amylase molecules, amylopectinmolecules or mixtures thereof that are linear and branched multimers ofglucose. Starch particles normally comprise crystalline and amorphousregions. Upon heating in solution, starch particles absorb water readilyand, upon gelatinization, the starch particles become highly osmoticallyactive. When returning to room temperature, starches gelatinize, but notrecrystalize. Thus, PVA can be mixed effectively with crystalline starchgranules to make a solution where the PVA is soluble. However, uponheating above the gelatinization point of the starch and recooling, thePVA would be forced to gel because of the competition for solvent withthe gelatinized starch which is more hygroscopic.

[0225] In another embodiment, the sequestration method involves the useof gel-based capsules, which upon a suitable trigger (for example,without limitation, pH, ionic concentration, temperature, radiation)release their encapsulated contents. In a further embodiment, thesequestration method involves the trapping of the active molecules in adegradable matrix. In preferred embodiments, a suitable biodegradablepolymer can be selected from the group including, but not limited to, apoly(L-lactide), a poly(D,L-lactide), a poly(lactide-co-glycolide), apoly(ε-caprolactone), a polyanhydride, a poly(beta-hydroxy butyrate), apoly(ortho ester) and a polyphosphazene.

[0226] In general, a suitable gellant in accordance with the preferredembodiments is a solute that is water soluble, has a higher affinity forwater than PVA. In preferred embodiments, a solid gellant or an aqueoussolution of gellant is added to an aqueous PVA solution. Typically PVAsolutions in the range of from about 1 weight % to about 50 weight % PVAare prepared by adding the desired amount of PVA to warm deionized waterwhile mixing. For example, a 20% PVA solution by weight is prepared bydissolving 20 g of PVA (100 kg/mole; 99.3+% hydrolyzed; J T Baker) in 80g of deionized water heated in a water bath to a temperature of greaterthan 90 degrees Celsius with continuous stirring for a minimum of 15minutes using a vortex mixer (VWR BRAND). The PVA solution obtained wassubstantially clear when fully dissolved and melted. The solution wasplaced in a covered container to avoid evaporation, optionally under amineral oil protective layer.

[0227] In one preferred embodiment, 1.4 g of 400 molecular weightpoly(ethylene glycol) (PEG400, Sigma Aldrich) was added gradually to 6.0g of 10% by weight PVA solution was stirred while stirring on a hotplate at 50 degrees Celsius and then the jar was shaken. After initiallyproducing an inhomogeneous solution whilst adding the material becamehomogeneous and rapidly opaque. The final gel was (by weight) 8% PVA,19% PEG 400 and 73% water. FIGS. 22A-22D show the product at four timedurations after the end of mixing: FIG. 22A, zero minutes; FIG. 22B, 15minutes; FIG. 22C, 2 hours, under a mineral oil protective layer; andFIG. 22D, one day, out of the jar.

[0228] In one preferred embodiment, a PVA hydrogel was prepared byadding 35.6 g of aqueous 10 wt % PVA solution to 18.7 g of aqueous 5.1 MNaCl while mixing, and the resulting mixture aggressively shaken. Thesolution was briefly inhomogeneous before becoming smooth andtransparent. Over the period of 16 hours the solution became graduallymore opaque and gelled. The final gel was 7 wt % PVA, 8 wt % NaCl and 85wt % water. FIGS. 23A-23E illustrate the PVA hydrogel prepared, showingthe product at five time durations after pouring into a covered dish anda flexible bag: FIG. 23A, zero minutes; FIG. 23B, 20 minutes; FIG. 23C,1 hour; FIG. 23D, 2 hours; and FIG. 23E, 17 hours. Note that thesolution was fluid for long enough that a circular shape was easilyformed. The solution was also cast in a flexible bag to demonstrate itsabilities for space filling applications in deformable environments.

[0229] In another embodiment, an aqueous 10 wt % PVA solution was placedin the larger barrel of a 4:1 ratio epoxy adhesive gun (3M).Poly(ethylene) glycol with a molecular weight of 400 g/mol was placed inthe smaller barrel. The resulting blend was delivered through a 3 inchstatic mixing nozzle (3M) into a mold held at room temperature. Theresulting mix had 8 wt % PVA, 20 wt % PEG 400 and 72 wt % water. Theresulting mixture was observed to gel inhomogeneously on delivery, butwith time resulted in a homogeneous opaque gel.

[0230] In a further preferred embodiment, a PVA hydrogel was prepared byadding NaCl to an aqueous 10 wt % PVA solution at about 95 degreesCelsius (FIG. 24A) while mixing to make a final concentration of 2MNaCl. After approximately 15 minutes, the resulting solution was smoothand homogeneous (FIG. 24B). The PVA solution obtained was poured intotwo jars, one of which was equilibrated at room temperature (FIG. 24C,15 minutes; FIG. 24E, one hour), the other of which was placed on shavedice (FIG. 24D, 15 minutes; FIG. 24F, one hour). The solution wasinitially cloudy, but homogeneous. After 1 hour, the gel cooled at roomtemperature was slightly cloudy, whereas the ice-cooled gel waspredominantly transparent.

[0231] The two samples were then stored at room temperature. The finalresulting gels after one month exhibited significant syneresis andvisually looked identical, but the rapidly cooled material appeared toturn opaque much faster.

[0232]FIGS. 25A-25F illustrate the PVA hydrogels of FIGS. 24A-24F afterstorage. FIG. 25A, cooled one hour at room temperature and stored 12hours at room temperature; FIG. 25B, cooled one hour on ice and stored12 hours at room temperature; FIG. 25C, cooled one hour at roomtemperature and stored one month at room temperature; FIG. 25D, cooledone hour on ice and stored one month at room temperature; FIG. 25E, thePVA gel of FIG. 25C, oriented to show water released due to syneresis;and FIG. 25F, the PVA gel of FIG. 25D, oriented to show water releaseddue to syneresis.

[0233] In preferred embodiments, the present invention provides a methodfor early treatment of joint disease by providing a polymer cushionformed in situ between load-bearing surfaces in the joint. In onepreferred embodiment, a PVA cushion is formed in situ within the hipjoint by dislocating the head of the femur, filling the exposed cavitywithin the joint with a fluid solution of PVA and gellant, replacing thehead of the femur and allowing the PVA solution to gel in situ. In anexample, illustrated in FIGS. 26A-26D, dry NaCl was added at a moderaterate to a 20 wt % aqueous PVA solution warmed in a water bath at about95 degrees Celsius with continuous stirring to make a 2.1 M NaClsolution. After 1 minute the resulting solution was smooth andmalleable, resembling taffy. Resulting solution was removed from themixer and placed in a chilled polyethylene liner from a Total HipReplacement (THR) system. The matching cobalt-chrome ball from the THRjoint inserted into the liner socket and allowed to stand for 1 hour atroom temperature. The mold was then placed in deionized water for afurther 1 hour, whereupon the ball was removed from the poly(ethylene)liner. At this point a thin, homogeneous and substantially blemish-freehemisphere of PVA was obtained. FIG. 26A shows the mold formed by thechilled polyethylene liner and the matching ball from a total hipreplacement joint. FIG. 26B shows the mold after filling the chilledpolyethylene liner with the PVA solution and putting the matching ballin place. FIG. 26C shows the molded PVA in the polyethylene liner afterone hour in air at room temperature followed by one hour in deionizedwater at room temperature. FIG. 26D shows the molded PVA product removedfrom the polyethylene liner.

[0234] Since the mechanism of gelation is chemically non-specific, it ispossible to use virtually any solute that has sufficient osmoticactivity to force PVA self-association. In a further preferredembodiment, the co-nonsolvency was exploited by using anaturally-occurring biocompatible material as a gellant to form a PVAgel. Chondroitin sulfate (CS, Now Foods Bloomingdale, Ind.) was used toinduce the gelation of polyvinyl alcohol. Aqueous 10 wt % PVA solutionwas prepared and stored at 60° C. until use. In a first example, warmchondroitin sulfate solution (˜80° C.) was added to the warm PVAsolution to generate a 5 wt % PVA, 7 wt % CS mixture. Upon cooling toroom temperature, the mixture formed a weak gel over a period of twodays that remained stable (FIG. 27A). In the second example, 600 mg CSwas added directly with continuous stirring to 10 ml of 10 wt % PVAsolution at 60 degrees Celsius. Compared to the previous example, themixture formed a much stiffer gel within minutes and remained stable(FIG. 27B). The success of inducing the gelation of PVA using abiocompatible material that is typically present in the joint spacesuggests the possibility of in situ joint repair or augmentation.

[0235] In a further example, serine, a common amino acid in the bloodstream of humans was dissolved in deionized water to produce a 30 wt %aqueous solution. The PVA solution and serine solution were thencombined in equal parts by volume at less than 90 degrees Celsius andmixed thoroughly, resulting in a solution of 10% PVA, 15% serine and 75%water. The solution stayed fluid while at the higher temperature but asit cooled it gradually gelled, producing a gel after about 1 hour.

[0236] In a preferred embodiment, the vinyl polymer solution mayinclude, without limitation, mixtures of vinyl polymers such aspolyvinyl alcohol and polyvinyl pyrollidone (PVP) or copolymers of PVP,as described in the European Patent specification EP 1229 873 B 1, theentire teachings of which are incorporated herein by reference.

[0237] In a preferred embodiment, the vinyl polymer solution may includeany mixture of components that form physical associations throughmanipulation of relative solvent quality.

[0238] In a preferred embodiment, the vinyl polymer solution may includea nano or microstructuring agent which can include nano andmicroparticulates such as clay or silica, charged or uncharged, and/ornanostructuring functionalized molecules such as POSS as describedherein before. These nano or microparticulates provide nucleation sitesthat accelerate or augment the gelation process. Preferred embodimentsof the present invention benefit from this recognition that nucleationsites provided by any particles of the appropriate size in the vinylpolymer solution augment gelation to result in a gel of the desiredmechanical properties.

[0239] Repair of Damaged Intervertebral Disks

[0240] In preferred embodiments, the method and dispenser of the presentinvention are used in the repair of a damaged intervertebral disk. FIG.33 is a schematic illustration of a midsagittal cross-section 900through two vertebrae, each vertebra having vertebral body 920 andspinous process 922; the two vertebral bodies enclosing anintervertebral disk 910 comprising annulus fibrosis 912, nucleuspulposus 914 and herniation 916; provided for orientation are verticalaxis 930, anterior-posterior axis 932 and arrow 938 indicating posterioraccess path to the herniation 916. FIG. 34 is a schematic illustrationof a transverse section 940 through a damaged intervertebral disk 910,showing spinous process 922 and transverse process 924 of an adjacentvertebra, annulus fibrosis 912, nucleus pulposus 914, the outline of thespinal cord 942 and herniation 916 protruding through defect 918.

[0241] Briefly, the damaged intervertebral disk is repaired byrespecting the herniated region, injecting a viscoelastic solution ofthe vinyl polymer hydrogel of the present invention to replace part orsubstantially all of the nucleolus pulposus material, controlling therate of gelation of the vinyl polymer hydrogel by methods describedherein before, and closing the injection site. The viscoelastic solutionof the vinyl polymer hydrogel can be injected through a defect in theannulus fibrosus at the site of the herniation, and/or through anotherpoint of the annulus fibrosus. The injection site and the defect can beclosed by modifying the local physical properties of the hydrogel by afurther application of gellant in situ, as described herein before.Alternatively, the injection site and the defect can be sealed andreinforced by the use of known medical devices to seal, reinforce orclose the injection site or other defect of the body cavity. Suitablesuch devices are disclosed in published International PatentApplications WO 01/12107 and WO 02/054978, which are hereby incorporatedby reference in their entirety.

[0242]FIG. 35 is an illustration of a step in a method for the repair ofa damaged intervertebral disk 910 in accordance with a preferredembodiment of the present invention, showing in transverse section 960 aspinous process 922 and a transverse process 924 of an adjacentvertebra, annulus fibrosis 912, nucleus pulposus 914, the outline of thespinal cord 942 and dispensing tube 934 introduced through defect 918,dispensing the viscoelastic mixture of gellant and vinyl polymer 952. Inpreferred embodiments, the local physical properties of the hydrogel areadjusted by an addition of a gellant through the same dispensing tube934 following the dispensing of the desired amount of viscoelasticmixture of gellant and vinyl polymer. The additional gellant can be thesame or different from the gellant initially mixed with the vinylpolymer solution.

[0243] In alternative embodiments, a sealer can be used to supplement orinstead of modifying the local physical properties of the hydrogel by anaddition of a gellant. FIG. 36 is a schematic illustration of a step inthe repair in accordance with a preferred embodiment of the presentinvention of a damaged intervertebral disk 910, showing in transversesection 962 a spinous process 922 and a transverse process 924 of anadjacent vertebra, annulus fibrosis 912, nucleus pulposus 914, theoutline of the spinal cord 942, defect 918, mixture of gellant and vinylpolymer 952, sealant 970, fixative 918, a fixative delivery instrument950. The sealant 970 is constructed from a material and is formed insuch a manner as to resist the passage of fluids and other materialsaround the sealant 970 and through the defect 918. The sealant 970 canbe constructed from one or any number of a variety of materialsincluding, but not limited to, polytetrafluoroethylene (PTFE), expandedpolytetrafluoroethylene (e-PTFE), NYLON™, MARLEX™, high densitypolyethylene, and/or collagen. See WO 02/054978, incorporated herein byreference. After placement of the sealant 970,fixative 918, such assutures or soft tissue anchors, are placed using fixative deliveryinstrument 950.

[0244] In alternative embodiments, a barrier can be used. FIG. 37 is aschematic illustration of a step in the repair in accordance with apreferred embodiment of the present invention of a damagedintervertebral disk 910, showing in transverse section 964 a spinousprocess 922 and a transverse process 924 of an adjacent vertebra,annulus fibrosis 912, nucleus pulposus 914, the outline of the spinalcord 942, defect 918, mixture of gellant and vinyl polymer 952, barrier974, fixative 972, and fixation delivery instrument 950. The barrier 974is preferably flexible in nature, and can be constructed from a wovenmaterial such as DACRON™ or NYLON™, a synthetic polyamide, polyester,polyethylene, and may be an expanded material such as e-PTFE.Alternatively, the barrier 974 can be a biologic material such ascollagen. The barrier 974 can be expandable such as, for example, aballoon or a hydrophilic material. See WO 02/054978.

[0245] If appropriate the viscoelastic solution of the present inventionis introduced at a site other than the defect. FIG. 38 is a schematicillustration of a step in the repair in accordance with a preferredembodiment of the present invention of a damaged intervertebral disk910, showing in transverse section 968 a spinous process 922 and atransverse process 924 of an adjacent vertebra, annulus fibrosis 912,the outline of the spinal cord 942, defect 918, mixture of gellant andvinyl polymer 952 substantially filling the space previously occupied bythe nucleus pulposus, barrier 976, and dispensing tube 934.

[0246] The claims should not be read as limited to the described orderor elements unless stated to that effect. Therefore, all embodimentsthat come within the scope and spirit of the following claims andequivalents thereto are claimed as the invention.

We claim:
 1. A method of making a vinyl polymer hydrogel having adesired physical property comprising the steps of: providing a vinylpolymer solution comprising a vinyl polymer dissolved in a firstsolvent; mixing the vinyl polymer solution with a gellant, wherein theresulting mixture has a higher Flory interaction parameter than thevinyl polymer solution; inducing gelation of the mixture of vinylpolymer solution and gellant; controlling the gelation rate to form aviscoelastic solution wherein workability is maintained for apredetermined period, thereby making a vinyl polymer hydrogel having thedesired physical property.
 2. The method of claim 1 wherein the step ofproviding a vinyl polymer solution includes the step of dissolving thevinyl polymer in the first solvent.
 3. The method of claim 1 furthercomprising the step of: heating the vinyl polymer solution to atemperature elevated above the melting point of the physicalassociations of the vinyl polymer.
 4. The method of claim 3 wherein thestep of mixing the vinyl polymer solution with a gellant precedes thestep of heating the vinyl polymer solution to a temperature elevatedabove the melting point of the physical associations of the vinylpolymer.
 5. The method of claim 1 wherein the desired physical propertyis at least one of light transmission, gravimetric swell ratio, shearmodulus, load modulus, loss modulus, storage modulus, dynamic modulus,compressive modulus, cross-linking and pore size.
 6. The method of claim1 wherein the desired physical property is physical cross-linking. 7.The method of claim 1 wherein the vinyl polymer is selected from thegroup consisting of polyvinyl alcohol, polyvinyl acetate, polyvinylpyrrollidone and mixtures thereof.
 8. The method of claim 7 wherein thevinyl polymer is highly hydrolyzed polyvinyl alcohol of about 50 kg/molto about 300 kg/mol molecular weight.
 9. The method of claim 7 whereinthe vinyl polymer is highly hydrolyzed polyvinyl alcohol of about 100kg/mol molecular weight.
 10. The method of claim 1 wherein the vinylpolymer solution is about 1 weight percent to about 50 weight percentsolution of polyvinyl alcohol based on the weight of the solution. 11.The method of claim 1 wherein the vinyl polymer solution is about 10weight percent to about 20 weight percent solution of polyvinyl alcoholbased on the weight of the solution.
 12. The method of claim 1 furthercomprising the step of contacting the viscoelastic solution with agellant.
 13. The method of claim 1 wherein the gellant is active whenmixed with the vinyl polymer solution.
 14. The method of claim 1 whereinthe gellant is inactive when mixed with the vinyl polymer solution. 15.The method of claim 14 wherein the step of inducing gelation of theviscoelastic solution includes the step of activating the gellant. 16.The method of claim 1, wherein the Flory interaction parameter of themixture of vinyl polymer solution and gellant ranges from 0.25 to 1.0.17. The method of claim 1 wherein the Flory interaction parameter of themixture is about 0.25 to about 0.5.
 18. The method of claim 1 whereinthe Flory interaction parameter of the mixture is at least 0.5.
 19. Themethod of claim 1 wherein the first solvent is selected from the groupconsisting of deionized water, dimethyl sulfoxide, an aqueous solutionof a C1 to C6 alcohol and mixtures thereof.
 20. The method of claim 1wherein the gellant is selected from the group consisting of salts,alcohols, polyols, amino acids, sugars, proteins, polysaccharides,aqueous solutions thereof, and mixtures thereof.
 21. The method of claim20 wherein the gellant is selected from the group consisting ofchondroitin sulfate, dermatan sulfate, hyaluronic acid, heparin sulfateand mixtures thereof.
 22. The method of claim 20 wherein the gellant isselected from the group consisting of biglycan, syndecan, keratocan,decorin, aggrecan and mixtures thereof.
 23. The method of claim 20wherein the gellant is an alkali metal salt.
 24. The method of claim 23wherein the alkali metal salt is sodium chloride.
 25. The method ofclaim 20 wherein the gellant is an aqueous solution of sodium chloridefrom about 1.5 molar to about 6.0 molar.
 26. The method of claim 20wherein the gellant is an aqueous solution of sodium chloride from about2.0 molar to about 6.0 molar
 27. The method of claim 20 wherein thegellant is an aqueous solution of an alcohol chosen from the groupsconsisting of methanol, ethanol, i-propanol, t-propanol, t-butanol andmixtures thereof.
 28. The method of claim 1 wherein the vinyl polymer isintroduced into an aqueous solution of a gellant.
 29. The method ofclaim 14 wherein the inactive gellant is activated by a trigger.
 30. Themethod of claim 14 wherein the inactive gellant is a macromolecule. 31.The method of claim 30 wherein the active gellant comprises fragments ofa macromolecule that are released by cleavage of the macromolecule. 32.The method of claim 31 wherein the cleavage of the macromolecule isenzymatic cleavage.
 33. The method of claim 30 wherein the macromoleculeis thermally denaturable.
 34. The method of claim 30 wherein themacromolecule is collagen.
 35. The method of claim 32 wherein themacromolecule is a physiological substrate of the enzyme.
 36. The methodof claim 35 wherein the macromolecule is selected from the groupconsisting of chondroitin sulfate, dermatan sulfate, keratan sulfate,hyaluronic acid, heparin, heparin sulfate and mixtures thereof and theenzyme is selected from the group consisting of chondroitinase ABC,chondroitinase AC, chondroitinase B, testicular hyaluronidase, hyaluronlyase, heparinase I/III and mixtures thereof.
 37. The method of claim 35wherein the macromolecule is selected from the group consisting ofbiglycan, syndecan, keratocan, decorin, aggrecan, perlecan,fibromodulin, versican, neurocan, brevican and mixtures thereof and theenzyme is selected from the group consisting of, but not limited to,aggrecanase and mixtures thereof.
 38. The method of claim 31 wherein thecleavage of the macromolecule is by irradiation with electromagneticradiation or particulate radiation.
 39. The method of claim 14 whereinthe inactive gellant is a bad solvent sequestered in a vesicle, aliposome, a micelle or a gel particle.
 40. The method of claim 39wherein the liposome is a phototriggerable diplasmalogen liposome. 41.The method of claim 39 wherein the liposome undergoes a phase transitionat about the body temperature of a mammal.
 42. The method of claim 39wherein the liposome comprises a mixture ofdipalmitoylphosphatidylcholine and dimyristoylphosphatidylcholine. 43The method of claim 39 wherein the gel particle releases its contentsupon undergoing a phase transition at about the body temperature of amammal.
 44. The method of claim 39 wherein the gel particle comprises apolymer selected from the group consisting of poly(N-isopropylacrylamide-co-acrylic acid), N-isopropylacrylamide, hyaluronic acid,pluronic and mixtures thereof.
 45. The method of claim 39 wherein thegel particle releases its contents upon undergoing degradation.
 46. Themethod of claim 1 wherein the gellant is more soluble than the vinylpolymer.
 47. The method of claim 1 further comprising the step ofprocessing the viscoelastic solution during the workability period. 48.The method of claim 47 wherein the step of processing includesinjecting, molding or calendaring.
 49. The method of claim 48 whereinthe viscoelastic solution is injected into an actual or potential spacein the body of a mammal.
 50. The method of claim 49 wherein theviscoelastic solution is injected into an intervertebral disk or anarticulated joint.
 51. The method of claim 47 wherein the step ofprocessing includes covering a burn or a wound.
 52. The method of claim1 wherein the viscoelastic solution further comprises one or morenon-gelling components.
 53. The method of claim 52 wherein theviscoelastic solution further comprises hyaluronic acid.
 54. The methodof claim 52 wherein the viscoelastic solution further comprisespolyacrylic acid.
 55. The method of claim 52 wherein the viscoelasticsolution further comprises a therapeutic agent.
 56. The method of claim1 wherein the vinyl polymer solution further comprises at least one ofnano or microparticulates to augment gelation.
 57. A physicallycross-linked gel produced by the method of: providing a vinyl polymersolution comprising a vinyl polymer dissolved in a first solvent;heating the vinyl polymer solution to a temperature elevated above themelting point of the physical associations of the vinyl polymer; mixingthe vinyl polymer solution with a gellant, wherein the resulting mixturehas a higher Flory interaction parameter than the vinyl polymersolution; inducing gelation of the mixture of vinyl polymer solution andgellant; controlling the gelation rate to form a viscoelastic solutionwherein workability is maintained for a predetermined period, therebymaking a physically cross-linked gel.
 58. The physically cross-linkedgel of claim 57 wherein the step of providing a vinyl polymer solutionincludes the step of dissolving the vinyl polymer in the first solvent.59. The physically cross-linked gel of claim 57 wherein the step ofmixing the vinyl polymer solution with a gellant precedes the step ofheating the vinyl polymer solution to a temperature elevated above themelting point of the physical associations of the vinyl polymer.
 60. Thephysically cross-linked gel of claim 57 wherein the vinyl polymer isselected from the group consisting of polyvinyl alcohol, polyvinylacetate, polyvinyl pyrrolidone and mixtures thereof.
 61. The physicallycross-linked gel of claim 60 wherein the vinyl polymer is highlyhydrolyzed polyvinyl alcohol of about 50 kg/mol to about 300 kg/molmolecular weight.
 62. The physically cross-linked gel of claim 60wherein the vinyl polymer is highly hydrolyzed polyvinyl alcohol ofabout 100 kg/mol molecular weight.
 63. The physically cross-linked gelof claim 57 wherein the vinyl polymer solution is about 1 weight percentto about 50 weight percent solution of polyvinyl alcohol based on theweight of the solution.
 64. The physically cross-linked gel of claim 57wherein the vinyl polymer solution is about 10 weight percent to about20 weight percent solution of polyvinyl alcohol based on the weight ofthe solution.
 65. The physically cross-linked gel of claim 57 furthercomprising the step of contacting the viscoelastic solution with agellant.
 66. The physically cross-linked gel of claim 57 wherein thegellant is active when mixed with the vinyl polymer solution.
 67. Thephysically cross-linked gel of claim 57 wherein the gellant is inactivewhen mixed with the vinyl polymer solution.
 68. The physicallycross-linked gel of claim 67 wherein the step of inducing gelation ofthe viscoelastic solution includes the step of activating the gellant.69. The physically cross-linked gel of claim 57, wherein the Floryinteraction parameter of the mixture of vinyl polymer solution andgellant ranges from 0.25 to 1.0.
 70. The physically cross-linked gel ofclaim 57 wherein the Flory interaction parameter of the mixture is about0.25 to about 0.5.
 71. The physically cross-linked gel of claim 57wherein the Flory interaction parameter of the mixture is at least 0.5.72. The physically cross-linked gel of claim 57 wherein the firstsolvent is selected from the group consisting of deionized water,dimethyl sulfoxide, an aqueous solution of a C1 to C6 alcohol andmixtures thereof.
 73. The physically cross-linked gel of claim 57wherein the gellant is selected from the group consisting of salts,alcohols, polyols, amino acids, sugars, proteins, polysaccharides,aqueous solutions thereof, and mixtures thereof.
 74. The physicallycross-linked gel of claim 73 wherein the gellant is selected from thegroup consisting of chondroitin sulfate, dermatan sulfate, hyaluronicacid, heparin sulfate and mixtures thereof.
 75. The physicallycross-linked gel of claim 73 wherein the gellant is selected from thegroup consisting of biglycan, syndecan, keratocan, decorin, aggrecan andmixtures thereof.
 76. The physically cross-linked gel of claim 73wherein the gellant is an alkali metal salt.
 77. The physicallycross-linked gel of claim 76 wherein the alkali metal salt is sodiumchloride.
 78. The physically cross-linked gel of claim 73 wherein thegellant is an aqueous solution of sodium chloride from about 1.5 molarto about 6.0 molar.
 79. The physically cross-linked gel of claim 73wherein the gellant is an aqueous solution of sodium chloride from about2.0 molar to about 6.0 molar
 80. The physically cross-linked gel ofclaim 73 wherein the gellant is an aqueous solution of an alcohol chosenfrom the groups consisting of methanol, ethanol, i-propanol, t-propanol,t-butanol and mixtures thereof.
 81. The physically cross-linked gel ofclaim 57 wherein the vinyl polymer is introduced into an aqueoussolution of a gellant.
 82. The physically cross-linked gel of claim 67wherein the inactive gellant is activated by a trigger.
 83. Thephysically cross-linked gel of claim 67 wherein the inactive gellant isa macromolecule.
 84. The physically cross-linked gel of claim 83 whereinthe active gellant comprises fragments of a macromolecule that arereleased by cleavage of the macromolecule.
 85. The physicallycross-linked gel of claim 84 wherein the cleavage of the macromoleculeis enzymatic cleavage.
 86. The physically cross-linked gel of claim 83wherein the macromolecule is thermally denaturable.
 87. The physicallycross-linked gel of claim 83 wherein the macromolecule is collagen. 88.The physically cross-linked gel of claim 85 wherein the macromolecule isa physiological substrate of the enzyme.
 89. The physically cross-linkedgel of claim 85 wherein the macromolecule is selected from the groupconsisting of chondroitin sulfate, dermatan sulfate, keratan sulfate,hyaluronic acid, heparin, heparin sulfate and mixtures thereof and theenzyme is selected from the group consisting of chondroitinase ABC,chondroitinase AC, chondroitinase B, testicular hyaluronidase, hyaluronlyase, heparinase I/III and mixtures thereof.
 90. The physicallycross-linked gel of claim 85 wherein the macromolecule is selected fromthe group consisting of biglycan, syndecan, keratocan, decorin,aggrecan, perlecan, fibromodulin, versican, neurocan, brevican andmixtures thereof and the enzyme is selected from the group consistingof, but not limited to, aggrecanase and mixtures thereof.
 91. Thephysically cross-linked gel of claim 85 wherein the cleavage of themacromolecule is by irradiation with electromagnetic radiation orparticulate radiation.
 92. The physically cross-linked gel of claim 82wherein the inactive gellant is a bad solvent sequestered in a vesicle,a liposome, a micelle or a gel particle.
 93. The physically cross-linkedgel of claim 92 wherein the liposome is a phototriggerable diplasmalogenliposome.
 94. The physically cross-linked gel of claim 92 wherein theliposome undergoes a phase transition at about the body temperature of amammal.
 95. The physically cross-linked gel of claim 92 wherein theliposome comprises a mixture of dipalmitoylphosphatidylcholine anddimyristoylphosphatidylcholine. 96 The physically cross-linked gel ofclaim 92 wherein the gel particle releases its contents upon undergoinga phase transition at about the body temperature of a mammal.
 97. Thephysically cross-linked gel of claim 92 wherein the gel particlecomprises a polymer selected from the group consisting ofpoly(NiPAAm-co-Aac), N-isopropylacrylamide, hyaluronic acid, pluronicand mixtures thereof.
 98. The physically cross-linked gel of claim 92wherein the gel particle releases its contents upon undergoingdegradation.
 99. The physically cross-linked gel of claim 57 wherein thegellant is more soluble than the vinyl polymer.
 100. The physicallycross-linked gel of claim 57 further comprising the step of processingthe viscoelastic solution during the workability period.
 101. Thephysically cross-linked gel of claim 100 wherein the step of processingincludes injecting, molding or calendaring.
 102. The physicallycross-linked gel of claim 100 wherein the viscoelastic solution isinjected into an actual or potential space in the body of a mammal. 103.The physically cross-linked gel of claim 102 wherein the viscoelasticsolution is injected into an intervertebral disk or an articulatedjoint.
 104. The physically cross-linked gel of claim 100 wherein thestep of processing includes covering a burn or a wound.
 105. Thephysically cross-linked gel of claim 57 wherein the viscoelasticsolution further comprises one or more non-gelling components.
 106. Thephysically cross-linked gel of claim 105 wherein the viscoelasticsolution further comprises hyaluronic acid.
 107. The physicallycross-linked gel of claim 105 wherein the viscoelastic solution furthercomprises polyacrylic acid.
 108. The physically cross-linked gel ofclaim 105 wherein the viscoelastic solution further comprises atherapeutic agent.
 109. The physically cross-linked gel of claim 57wherein the vinyl polymer solution is an aqueous solution of about 10weight percent to about 30 weight percent polyvinyl alcohol based on theweight of the solution.
 110. The physically cross-linked gel of claim 57wherein the gellant is an aqueous solution of sodium chloride from about1.5 molar to about 6.0 molar.
 111. The physically cross-linked gel ofclaim 57 wherein the gellant is an aqueous solution of sodium chloridefrom about 1.5 molar to about 3.0 molar.
 112. The physicallycross-linked gel of claim 57 wherein the gellant is an aqueous solutionof sodium chloride from about 1.75 molar to about 6.0 molar.
 113. Aphysically cross-linked hydrogel substantially free of chemicalcrosslinkers.
 114. A physically cross-linked hydrogel comprising atleast about 10 weight percent polyvinyl alcohol solution gelled byimmersion in about 2 to about 3 molar sodium chloride wherein thehydrogel is about 14 percent to about 21 percent physically crosslinked.115. The physically cross-linked hydrogel of claim 114 wherein the gelcomprises about 12 to about 29 percent polyvinyl alcohol.
 116. Thephysically cross-linked hydrogel of claim 114 wherein the vinyl polymersolution contains one or more non-gelling components.
 117. Thephysically cross-linked hydrogel of claim 116 further comprisinghyaluronic acid.
 118. The physically cross-linked hydrogel of claim 116further comprising polyacrylic acid.
 119. The physically crosslinked gelof claim 116 further comprising a therapeutic agent.
 120. The method ofclaim 1 wherein gelation is stopped by reducing the local concentrationof the gellant.
 121. The method of claim 120 wherein the localconcentration of the gellant is reduced by diffusion.
 122. A kit forproviding vinyl polymer hydrogels to a region of interest comprising: acontainer of a vinyl polymer; a container of a first solvent; acontainer of a gellant; and a delivery device.
 123. The kit of claim 122further comprising instructions for use.
 124. The kit of claim 122wherein the delivery device is a dispenser.
 125. The kit of claim 122wherein the dispenser further comprises a first chamber.
 126. The kit ofclaim 122 wherein the dispenser further comprises a second chamber. 127.The kit of claim 122 wherein the dispenser further comprises a mixingchamber.
 128. The kit of claim 122 wherein the dispenser furthercomprises a dispensing tube.
 129. The kit of claim 122 wherein thedispenser further comprises at least one of a heater and cooler incommunication with the dispensing chamber.
 130. The kit of claim 122wherein the dispenser further comprises at least one of a heater andcooler in communication with the mixing chamber.
 131. The kit of 122further comprising a temperature controller.
 132. A method for sealing adefect in a soft tissue in a region of interest comprising the steps of:providing a first portion of a vinyl polymer hydrogel manufactured by:providing a vinyl polymer solution comprising a vinyl polymer dissolvedin a first solvent; heating the vinyl polymer solution to a temperatureelevated above the melting point of the physical associations of thevinyl polymer; mixing the vinyl polymer solution with a gellant,whereing the resulting mixture has a higher Flory interaction parameterthan the vinyl polymer solution; inducing gelation of the mixture ofvinyl polymer solution and gellant; controlling the gelation rate toform a viscoelastic solution wherein workability is maintained for apredetermined period, thereby making a vinyl polymer hydrogel having thedesired physical property; and providing a second portion of the vinylpolymer hydrogel having a high concentration level of the hydrogel. 133.An injectable hydrogel for nucleus pulposus augmentation manufactured bythe method comprising the step of: providing a first portion of a vinylpolymer hydrogel manufactured by: providing a vinyl polymer solutioncomprising a vinyl polymer dissolved in a first solvent; heating thevinyl polymer solution to a temperature elevated above the melting pointof the physical associations of the vinyl polymer; mixing the vinylpolymer solution with a gellant, whereing the resulting mixture has ahigher Flory interaction parameter than the vinyl polymer solution;inducing gelation of the mixture of vinyl polymer solution and gellant;controlling the gelation rate to form a viscoelastic solution whereinworkability is maintained for a predetermined period, thereby making avinyl polymer hydrogel having the desired physical property; andproviding a second portion of the vinyl polymer hydrogel having a highconcentration level of the hydrogel.